Work around flaky TextTrackList tests that will be stable with Oilpan

Work around flaky TextTrackList tests that will be stable with Oilpan

This test calls document.createElement('video').textTracks and has no
reference to the video element. The TextTrackList doesn't keep its
owner HTMLMediaElement alive, so if the owner is garbage collected,
TextTrackList::executionContext() returns null. That in turn makes
EventTarget::dispatchEvent() do nothing, and thus the assert_true(ran)
fails because the event handler was in fact never run.

The owner is a RawPtrWillBeMember<HTMLMediaElement>, so this problem
will not occur with Oilpan, and does not seem worth addressing
pre-Oilpan unless there's evidence of this happening in the wild.

BUG=503852
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197907

[philipj_slow, 2015-06-26 10:46:51]

The CQ bit was checked by philipj@opera.com to run a CQ dry run

[commit-bot: I haz the power, 2015-06-26 10:47:01]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1213813002/1

[commit-bot: I haz the power, 2015-06-26 11:44:09]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-26 11:44:09]

Dry run: This issue passed the CQ dry run.

[philipj_slow, 2015-06-26 11:50:22]

philipj@opera.com changed reviewers:
+ fs@opera.com

[philipj_slow, 2015-06-26 11:50:22]

PTAL at this non-fix :)

[fs, 2015-06-26 11:54:50]

LGTM =(

[philipj_slow, 2015-06-26 11:58:47]

The CQ bit was checked by philipj@opera.com

[commit-bot: I haz the power, 2015-06-26 11:58:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1213813002/1

[commit-bot: I haz the power, 2015-06-26 12:02:05]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197907