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

Review URL: https://codereview.chromium.org/617293002

Cr-Commit-Position: refs/heads/master@{#297676}