Stabilize and re-enable TabCaptureApiPixelTest.EndToEnd.

Stabilize and re-enable TabCaptureApiPixelTest.EndToEnd.

This takes the lessons learned from stabilizing the
CastStreamingApiTest.EndToEnd test and rewrites much of the
TabCaptureApiPixelTest.EndToEnd JavaScript code to match.  In addition,
the test is now split into separate browser test fixtures to
differentiate between failures due to Chromium commits vs. WebRTC rolls.

BUG=367349, 396413, 406051
Committed: https://crrev.com/5dd13bd3a186330de2fea378f0c8b190b96a0515
Cr-Commit-Position: refs/heads/master@{#304335}

[miu, 2014-11-10 23:09:03]

Patchset #1 (id:1) has been deleted

[miu, 2014-11-11 00:24:18]

miu@chromium.org changed reviewers:
+ hubbe@chromium.org

[miu, 2014-11-11 00:24:18]

hubbe: PTAL.

[hubbe, 2014-11-15 01:15:14]

lgtm

[miu, 2014-11-15 01:15:59]

The CQ bit was checked by miu@chromium.org

[commit-bot: I haz the power, 2014-11-15 01:16:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/713623002/20001

[commit-bot: I haz the power, 2014-11-15 02:10:26]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2014-11-15 02:14:20]

Patchset 1 (id:??) landed as
https://crrev.com/5dd13bd3a186330de2fea378f0c8b190b96a0515
Cr-Commit-Position: refs/heads/master@{#304335}