This change introduces the HTML-based UI for the 2D Content window in Firefox
Reality for Desktop, accessed via the --fxr command line parameter.

Backed out for browser_parsable_css.js failures (custom properties issues)

backout: https://hg.mozilla.org/integration/autoland/rev/787d1f05a316abb2c26995d031a51affb439287b

push: https://treeherder.mozilla.org/#/jobs?repo=autoland&group_state=expanded&selectedJob=265061909&searchStr=windows%2Cbrowser-chrome&revision=e16dda7ba4bf9bc7674ca994397b69c1dd9f89bb

failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=265061909&repo=autoland&lineNumber=2323

[task 2019-09-05T03:02:35.631Z] 03:02:35 INFO - Console message: [JavaScript Warning: "Unknown pseudo-class or pseudo-element ‘-moz-svg-foreign-content’. Ruleset ignored due to bad selector." {file: "resource://gre/res/svg.css?always-parse-css-0.5908537391127116" line: 69}]
[task 2019-09-05T03:02:35.632Z] 03:02:35 INFO - Console message: [JavaScript Warning: "Unknown pseudo-class or pseudo-element ‘-moz-svg-text’. Ruleset ignored due to bad selector." {file: "resource://gre/res/svg.css?always-parse-css-0.5908537391127116" line: 84}]
[task 2019-09-05T03:02:35.632Z] 03:02:35 INFO - Console message: [JavaScript Warning: "Unknown pseudo-class or pseudo-element ‘-moz-svg-marker-anon-child’. Ruleset ignored due to bad selector." {file: "resource://gre/res/svg.css?always-parse-css-0.5908537391127116" line: 93}]
[task 2019-09-05T03:02:35.633Z] 03:02:35 INFO - Console message: [JavaScript Warning: "Expected media feature name but found ‘-moz-windows-default-theme’." {file: "jar:file:///Z:/task_1567646803/build/application/firefox/browser/features/formautofill@mozilla.org.xpi!/chrome/res/autocomplete-item.css?always-parse-css-0.5908537391127116" line: 12}]
[task 2019-09-05T03:02:35.633Z] 03:02:35 INFO - Buffered messages finished
[task 2019-09-05T03:02:35.633Z] 03:02:35 INFO - TEST-UNEXPECTED-FAIL | browser/base/content/test/static/browser_parsable_css.js | custom property --dusk is not referenced -
[task 2019-09-05T03:02:35.633Z] 03:02:35 INFO - Stack trace:
[task 2019-09-05T03:02:35.633Z] 03:02:35 INFO - chrome://mochikit/content/browser-test.js:test_ok:1580
[task 2019-09-05T03:02:35.633Z] 03:02:35 INFO - chrome://mochitests/content/browser/browser/base/content/test/static/browser_parsable_css.js:checkAllTheCSS:507
[task 2019-09-05T03:02:35.633Z] 03:02:35 INFO - chrome://mochikit/content/browser-test.js:Tester_execTest/<:1350
[task 2019-09-05T03:02:35.633Z] 03:02:35 INFO - chrome://mochikit/content/browser-test.js:Tester_execTest:1385
[task 2019-09-05T03:02:35.633Z] 03:02:35 INFO - chrome://mochikit/content/browser-test.js:nextTest/<:1213
[task 2019-09-05T03:02:35.633Z] 03:02:35 INFO - chrome://mochikit/content/tests/SimpleTest/SimpleTest.js:SimpleTest.waitForFocus/waitForFocusInner/focusedOrLoaded/<:805

https://hg.mozilla.org/mozilla-central/rev/965138a0dee1

Clearing needinfo, fix checked in