Replaces cricket::VideoFrame::SetToBlack and
cricket::WebRtcVideoFrame::InitToBlack, which are deleted.
Refactors the black frame logic in VideoBroadcaster, and a few of the
tests.
BUG=webrtc:5682
Review-Url: https://codereview.webrtc.org/2029273004
Cr-Commit-Position: refs/heads/master@{#13063}