If the track is disabled, replace incoming frames by black frames. Affects local rendering of disabled tracks.
Also intended to replace the similar logic in
WebRtcVideoChannel2::WebRtcVideoSendStream::InputFrame, once
VideoRtpSender is hooked up as a renderer for the track.
BUG=webrtc:5426
Review URL: https://codereview.webrtc.org/1575223003
Cr-Commit-Position: refs/heads/master@{#11297}