Introduce NullVideoSink for test classes.

Similar to NullAudioSink, but I decided not to use FakeAudioWorker
as a generic fake worker since it adds more complexity then we need
for the video renderer sink interface.  I also didn't want to add
pause support or stop callback support to it.

BUG=473424
TEST=new unittest, forthcoming pipeline changes.

Review URL: https://codereview.chromium.org/1116473002

Cr-Commit-Position: refs/heads/master@{#327450}
8 files changed