New static method I420Buffer::SetToBlack.

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-Original-Commit-Position: refs/heads/master@{#13063}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 663f9e2ddc86e813f6db04ba2cf5ac1ed9e7ef67
12 files changed