This patch adds the following functionality to the CSSStyleSheet WebIDL API
under the feature flag layout.css.constructable-stylesheets.enabled:

• constructor()
• replace()
• replaceSync()

replace() and replaceSync() are currently stubs that do nothing.

Backed out changeset 6e675d6399ce for causing failures in layout/style tests.

Backout link: https://hg.mozilla.org/integration/autoland/rev/bf5366280fcaeb6deb5720a8e5b0225c602bb751

Push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&tochange=bf5366280fcaeb6deb5720a8e5b0225c602bb751&fromchange=6e675d6399cee28f204b3501de3475ea3febd12c&searchStr=%2814&selectedJob=283092316

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=283092316&repo=autoland&lineNumber=3279

[task 2019-12-31T23:29:48.511Z] 23:29:48 INFO - TEST-OK | layout/style/test/test_mq_hover_and_pointer.html | took 1250ms
[task 2019-12-31T23:29:48.539Z] 23:29:48 INFO - GECKO(1227) | [Parent 1227, Main Thread] WARNING: we only accept nsIURI interface type, patch welcome: file /builds/worker/workspace/build/src/dom/ipc/PropertyBagUtils.cpp, line 112
[task 2019-12-31T23:29:48.668Z] 23:29:48 INFO - GECKO(1227) | [Child 1335, Main Thread] WARNING: Trying to request nsIHttpChannel from DocumentChannelChild, this is likely broken: file /builds/worker/workspace/build/src/netwerk/ipc/DocumentChannelChild.cpp, line 63
[task 2019-12-31T23:29:48.716Z] 23:29:48 INFO - TEST-START | layout/style/test/test_mq_prefers_reduced_motion_dynamic.html
[task 2019-12-31T23:29:48.844Z] 23:29:48 INFO - GECKO(1227) | [Parent 1227, Main Thread] WARNING: we only accept nsIURI interface type, patch welcome: file /builds/worker/workspace/build/src/dom/ipc/PropertyBagUtils.cpp, line 112
[task 2019-12-31T23:29:49.046Z] 23:29:49 INFO - GECKO(1227) | [Child 1335, Main Thread] WARNING: Trying to request nsIHttpChannel from DocumentChannelChild, this is likely broken: file /builds/worker/workspace/build/src/netwerk/ipc/DocumentChannelChild.cpp, line 63
[task 2019-12-31T23:29:49.131Z] 23:29:49 INFO - TEST-INFO | started process screentopng
[task 2019-12-31T23:29:49.595Z] 23:29:49 INFO - TEST-INFO | screentopng: exit 0
[task 2019-12-31T23:29:49.595Z] 23:29:49 INFO - Buffered messages logged at 23:29:49
[task 2019-12-31T23:29:49.597Z] 23:29:49 INFO - add_task | Entering test
[task 2019-12-31T23:29:49.598Z] 23:29:49 INFO - Buffered messages finished
[task 2019-12-31T23:29:49.599Z] 23:29:49 INFO - TEST-UNEXPECTED-FAIL | layout/style/test/test_mq_prefers_reduced_motion_dynamic.html | Does not matches prefers-reduced-motion: reduced) when the system sets prefers-reduced-motion false
[task 2019-12-31T23:29:49.600Z] 23:29:49 INFO - SimpleTest.ok@SimpleTest/SimpleTest.js:277:18
[task 2019-12-31T23:29:49.600Z] 23:29:49 INFO - @layout/style/test/test_mq_prefers_reduced_motion_dynamic.html:45:5
[task 2019-12-31T23:29:49.600Z] 23:29:49 INFO - asyncnextTick/<@SimpleTest/SimpleTest.js:1824:34
[task 2019-12-31T23:29:49.600Z] 23:29:49 INFO - nextTick@SimpleTest/SimpleTest.js:1840:11
[task 2019-12-31T23:29:49.600Z] 23:29:49 INFO - setTimeout handlerSimpleTest_setTimeoutShim@SimpleTest/SimpleTest.js:689:43
[task 2019-12-31T23:29:49.600Z] 23:29:49 INFO - add_task@SimpleTest/SimpleTest.js:1784:17
[task 2019-12-31T23:29:49.600Z] 23:29:49 INFO - @layout/style/test/test_mq_prefers_reduced_motion_dynamic.html:39:9
[task 2019-12-31T23:29:49.600Z] 23:29:49 INFO - Not taking screenshot here: see the one that was previously logged

Re-landed changeset 6e675d6399ce since the failures listed above actually started below it and we didn't see that both because of lack of coverage for that particular test.

https://hg.mozilla.org/mozilla-central/rev/8c36ba3d818d