Revert of No longer requiring pixel output for WebRTC tests.

Revert of No longer requiring pixel output for WebRTC tests. (patchset #4 id:60001 of https://codereview.chromium.org/611953004/)

Reason for revert:
Turns out this change makes video+canvas grabbing produce junk; this is not good since our tests depend on that working. Reverting.

Original issue's description:
> No longer requiring pixel output for WebRTC tests.
>
> We used to require pixel output for our canvas-scraping, but that
> appears to no longer be required. This speeds up the test 3x on my
> win machine, from 30 seconds to about 10.
>
> The content_browsertests on ChromeOS will still enable pixel
> output since it seems the requirements there are different - video
> tags will not work unless that is enabled, it seems. I think
> that's fine since our original problem was with the browser_tests
> anyway, and I think the content_browsertests are probably fast
> enough on ChromeOS anyway (whereas the browser_tests weren't).
>
> BUG=396505
>
> Committed: https://crrev.com/e6a342dcab57c3a2de1fd21301f37fdf3f8ad070
> Cr-Commit-Position: refs/heads/master@{#297654}

TBR=tommi@chromium.org,xians@google.com
NOTREECHECKS=true
NOTRY=true
BUG=396505
Committed: https://crrev.com/b8c01fc3dc165b8d3d6aea166776f07b25f06b04
Cr-Commit-Position: refs/heads/master@{#297800}

[phoglund_chromium, 2014-10-02 10:57:54]

Created Revert of No longer requiring pixel output for WebRTC tests.

[commit-bot: I haz the power, 2014-10-02 10:58:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/619003003/1

[commit-bot: I haz the power, 2014-10-02 10:59:58]

Committed patchset #1 (id:1) as bd59c3f3271e46ffd6f76893da2bd2803297204d

[commit-bot: I haz the power, 2014-10-02 11:00:33]

Patchset 1 (id:??) landed as
https://crrev.com/b8c01fc3dc165b8d3d6aea166776f07b25f06b04
Cr-Commit-Position: refs/heads/master@{#297800}