Requested by Yoric in https://github.com//servo/webrender/issues/3555

https://treeherder.mozilla.org/#/jobs?repo=try&revision=2b72640d7a9b8e4c38ba7862b898c5207ca2ac84

Depends on D20460

Backed out for failures on webdriver/tests/new_session/invalid_capabilities.py

backout: https://hg.mozilla.org/integration/autoland/rev/31bb666768271631710ac361e5625e3492cc82b3

push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&revision=27882e7ca2a9fb4a48238749ce4aa1447aa6c507&searchStr=wd

failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=229439857&repo=autoland&lineNumber=136059

[task 2019-02-20T15:46:07.686Z] 15:46:07 INFO - STDOUT: <undetermined location>
[task 2019-02-20T15:46:07.687Z] 15:46:07 INFO - STDOUT: Module already imported so cannot be rewritten: mozlog
[task 2019-02-20T15:46:07.687Z] 15:46:07 INFO - STDOUT: Module already imported so cannot be rewritten: tests.support.fixtures
[task 2019-02-20T15:46:07.689Z] 15:46:07 INFO - STDOUT: -- Docs: http://doc.pytest.org/en/latest/warnings.html
[task 2019-02-20T15:46:07.689Z] 15:46:07 INFO - STDOUT: =============== 1 failed, 205 passed, 2 warnings in 5.61 seconds ===============
[task 2019-02-20T15:46:07.705Z] 15:46:07 INFO -
[task 2019-02-20T15:46:07.706Z] 15:46:07 INFO - TEST-PASS | /webdriver/tests/new_session/invalid_capabilities.py | test_invalid_capabilites[None]
[task 2019-02-20T15:46:07.706Z] 15:46:07 INFO - TEST-PASS | /webdriver/tests/new_session/invalid_capabilities.py | test_invalid_capabilites[1]
[task 2019-02-20T15:46:07.706Z] 15:46:07 INFO - TEST-PASS | /webdriver/tests/new_session/invalid_capabilities.py | test_invalid_capabilites[{}]
[task 2019-02-20T15:46:07.707Z] 15:46:07 INFO - TEST-UNEXPECTED-FAIL | /webdriver/tests/new_session/invalid_capabilities.py | test_invalid_capabilites[value3] - assert 200 == 400
[task 2019-02-20T15:46:07.707Z] 15:46:07 INFO - new_session = <function new_session at 0x7f24e287e050>, value = []
[task 2019-02-20T15:46:07.707Z] 15:46:07 INFO -
[task 2019-02-20T15:46:07.708Z] 15:46:07 INFO - @pytest.mark.parametrize("value", [None, 1, "{}", []])
[task 2019-02-20T15:46:07.708Z] 15:46:07 INFO - def test_invalid_capabilites(new_session, value):
[task 2019-02-20T15:46:07.708Z] 15:46:07 INFO - response, _ = new_session({"capabilities": value})
[task 2019-02-20T15:46:07.709Z] 15:46:07 INFO - > assert_error(response, "invalid argument")
[task 2019-02-20T15:46:07.709Z] 15:46:07 INFO -
[task 2019-02-20T15:46:07.709Z] 15:46:07 INFO - _ = {'capabilities': {'acceptInsecureCerts': False, 'browserName': 'firefox', 'browserVersion': '62.0.3', 'moz:accessibilityChecks': False, ...}, 'sessionId': 'd0c27cf4-d894-4e77-82b4-7f0ae88883ee'}
[task 2019-02-20T15:46:07.710Z] 15:46:07 INFO - new_session = <function new_session at 0x7f24e287e050>
[task 2019-02-20T15:46:07.710Z] 15:46:07 INFO - response = <Responsetatus=200 body={"value": {"sessionId": "d0c27cf4-d894-4e77-82b4-7f0ae...moz:processID": 6812, "pageLoadStrategy": "normal", "platformName": "linux"}}}>
[task 2019-02-20T15:46:07.711Z] 15:46:07 INFO - value = []
[task 2019-02-20T15:46:07.711Z] 15:46:07 INFO -
[task 2019-02-20T15:46:07.712Z] 15:46:07 INFO - tests/web-platform/tests/webdriver/tests/new_session/invalid_capabilities.py:12:
[task 2019-02-20T15:46:07.712Z] 15:46:07 INFO - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
[task 2019-02-20T15:46:07.712Z] 15:46:07 INFO -
[task 2019-02-20T15:46:07.713Z] 15:46:07 INFO - response = <Responsetatus=200 body={"value": {"sessionId": "d0c27cf4-d894-4e77-82b4-7f0ae...moz:processID": 6812, "pageLoadStrategy": "normal", "platformName": "linux"}}}>
[task 2019-02-20T15:46:07.713Z] 15:46:07 INFO - error_code = 'invalid argument'
[task 2019-02-20T15:46:07.714Z] 15:46:07 INFO -
[task 2019-02-20T15:46:07.714Z] 15:46:07 INFO - def assert_error(response, error_code):
[task 2019-02-20T15:46:07.714Z] 15:46:07 INFO - """
[task 2019-02-20T15:46:07.715Z] 15:46:07 INFO - Verify that the provided webdriver.Response instance described
[task 2019-02-20T15:46:07.715Z] 15:46:07 INFO - a valid error response as defined by dfn-send-an-error and
[task 2019-02-20T15:46:07.716Z] 15:46:07 INFO - the provided error code.
[task 2019-02-20T15:46:07.717Z] 15:46:07 INFO -
[task 2019-02-20T15:46:07.717Z] 15:46:07 INFO - :param response: webdriver.Response instance.
[task 2019-02-20T15:46:07.718Z] 15:46:07 INFO - :param error_code: String value of the expected error code
[task 2019-02-20T15:46:07.718Z] 15:46:07 INFO - """
[task 2019-02-20T15:46:07.719Z] 15:46:07 INFO - > assert response.status == errors[error_code]
[task 2019-02-20T15:46:07.719Z] 15:46:07 INFO - E assert 200 == 400
[task 2019-02-20T15:46:07.719Z] 15:46:07 INFO - E + where 200 = <Responsetatus=200 body={"value": {"sessionId": "d0c27cf4-d894-4e77-82b4-7f0ae...moz:processID": 6812, "pageLoadStrategy": "normal", "platformName": "linux"}}}>.status
[task 2019-02-20T15:46:07.719Z] 15:46:07 INFO -
[task 2019-02-20T15:46:07.720Z] 15:46:07 INFO - error_code = 'invalid argument'
[task 2019-02-20T15:46:07.720Z] 15:46:07 INFO - response = <Responsetatus=200 body={"value": {"sessionId": "d0c27cf4-d894-4e77-82b4-7f0ae...moz:processID": 6812, "pageLoadStrategy": "normal", "platformName": "linux"}}}>
[task 2019-02-20T15:46:07.721Z] 15:46:07 INFO -
[task 2019-02-20T15:46:07.721Z] 15:46:07 INFO - tests/web-platform/tests/webdriver/tests/support/asserts.py:50: AssertionError
[task 2019-02-20T15:46:07.767Z] 15:46:07 INFO - ..........................................................................................................................................................................................................
[task 2019-02-20T15:46:07.767Z] 15:46:07 INFO - TEST-OK | /webdriver/tests/new_session/invalid_capabilities.py | took 5740ms

I saw that failure in my try push too but thought it was unrelated. Most likely I screwed up the rebase of serde_derive. Anyway, :whimboo interpreted the failure for me, said it was due to the cookie endpoint not deserializing properly, involving the structure at https://searchfox.org/mozilla-central/rev/4587d146681b16ff9878a6fdcba53b04f76abe1d/testing/webdriver/src/command.rs#383

I'll try redoing the rebase and/or narrowing down the source of the problem somehow.

So it wasn't a rebasing error, it's just that the new serde/serde_derive breaks the webdriver test.

Here's a try push with just 1.0.88 and without our special sauce patch for the enums:
https://treeherder.mozilla.org/#/jobs?repo=try&group_state=expanded&revision=573a88c4591b4adc41434bbce466943af5905f09

Has the same failure. I'll spin off another bug for this and try to bisect serde a bit.

https://treeherder.mozilla.org/#/jobs?repo=try&group_state=expanded&revision=354f78c59659da091aa9134ca7f409604ad61cfd

Looking green after bug 1529976 is in. I'll wait until that hits m-c before relanding this.

Backed out 2 changesets (bug 1529117) for build bustage

Log:
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=230740337&repo=autoland&lineNumber=222

Push with failures:
https://treeherder.mozilla.org/#/jobs?repo=autoland&revision=82cfcc2f5fac53e2c989bff17e84e8fa0f7a1c85

Backout:
https://hg.mozilla.org/integration/autoland/rev/d819234834aad12e3388ceb0b09dfb6c8cdd3da9

There are some r+ patches which didn't land and no activity in this bug for 2 weeks.
:kats, could you have a look please?

Waiting on dependencies

Rebased:

https://treeherder.mozilla.org/#/jobs?repo=try&group_state=expanded&revision=4ab72402808cb8831af92c5c912c9503c5d9d830
https://treeherder.mozilla.org/#/jobs?repo=try&group_state=expanded&revision=003c983dc52843f332c4117b4bee6689de0964a4

First one breaks standalone webrender; the second one has a fix for that

https://hg.mozilla.org/mozilla-central/rev/9765d149f513
https://hg.mozilla.org/mozilla-central/rev/5a879ab3ad21