Replace VideoStream::SizeCallback with VideoStream::Observer interface.

The new Observer interface will allow to notify ClientSession when a
video frame is sent, which will allow to provide the client with the
frame_id->event_timestamp mapping.

BUG=621691

Review-Url: https://codereview.chromium.org/2083843002
Cr-Commit-Position: refs/heads/master@{#401214}