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}