Safari 11 MediaSource failures

[joeyparrish]

Safari 11 fails several MediaSourceEngine tests. We need to:

1. look into the test failures
2. file additional bugs against Apple (if necessary)
3. improve the Safari MediaSource polyfills (if possible)

[joeyparrish]

Safari 11.0.0 (Mac OS X 10.11.6) MediaSourceEngine removes segments FAILED
	Expected 15 to be close to 10.
	test/media/media_source_engine_integration.js:142:57
	promiseReactionJob@[native code]
	Expected 15 to be close to 20.
	test/media/media_source_engine_integration.js:143:58
	promiseReactionJob@[native code]
	Expected 25 to be 20.
	test/media/media_source_engine_integration.js:146:50
	promiseReactionJob@[native code]
	Expected 5 to be close to 10.
	test/media/media_source_engine_integration.js:147:58
	promiseReactionJob@[native code]
Safari 11.0.0 (Mac OS X 10.11.6) MediaSourceEngine trims content at appendWindowEnd FAILED
	Expected 6.916666666666667 to be close to 10.
	test/media/media_source_engine_integration.js:332:57
	promiseReactionJob@[native code]
	Expected 6.916666666666667 to be close to 18, 1.
	test/media/media_source_engine_integration.js:335:57
	promiseReactionJob@[native code]
Safari 11.0.0 (Mac OS X 10.11.6): Executed 51 of 1287 (2 FAILED) (skipped 1236) (2.357 secs / 1.862 secs)

[joeyparrish]

The failure of the "trims content at appendWindowEnd" test is caused by a bug we've been working around in Safari 10: https://bugs.webkit.org/show_bug.cgi?id=165342

We can apply the same polyfill to Safari 11.

[joeyparrish]

New MediaSource bug filed against Safari 11: https://bugs.webkit.org/show_bug.cgi?id=177884

[joeyparrish]

Cherry-picked to v2.2.3.