Speeding up WebRTC content_browsertests by not rendering video.

Speeding up WebRTC content_browsertests by not rendering video.

I noticed signficant speed-up on my machine by not renderering the
video element on the page, and the tests don't seem to care (they can
still dig out pixels with video+canvas grabbing). It would be better
if we could remove EnablePixelOutput and not render anything at all,
but unfortunately that seems to break video+canvas grabbing on non-GPU
bots.

BUG=417756
Committed: https://crrev.com/5d572d101ed29547716bca58a46f3e32d2e40fed
Cr-Commit-Position: refs/heads/master@{#297676}

[phoglund_chromium, 2014-10-01 15:00:23]

phoglund@chromium.org changed reviewers:
+ kjellander@chromium.org

[kjellander_chromium, 2014-10-01 15:51:05]

lgtm

[phoglund_chromium, 2014-10-01 16:00:24]

The CQ bit was checked by phoglund@chromium.org

[commit-bot: I haz the power, 2014-10-01 16:01:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/617293002/1

[commit-bot: I haz the power, 2014-10-01 18:11:49]

Committed patchset #1 (id:1) as 992dd4d28c10dcab4df83d65dac604fdeae7ceca

[commit-bot: I haz the power, 2014-10-01 18:13:34]

Patchset 1 (id:??) landed as
https://crrev.com/5d572d101ed29547716bca58a46f3e32d2e40fed
Cr-Commit-Position: refs/heads/master@{#297676}