Replaces render_time_ms_, but old accessors are kept for
compatibility.
Also short-circuit timestamp translation in
WebRtcVideoChannel2::WebRtcVideoSendStream::OnFrame.
BUG=webrtc:5682, webrtc:5740
Review-Url: https://codereview.webrtc.org/2282713002
Cr-Commit-Position: refs/heads/master@{#14062}