Passing transport_frame_id() to VideoSink will allow to identify incoming video
frames, which will make it possible to correlate video frames on the
sender and on the receiver.
BUG=chromium:621691
R=mflodman@webrtc.org, stefan@webrtc.org
Review URL: https://codereview.webrtc.org/2088953002 .
Cr-Commit-Position: refs/heads/master@{#13596}