This cl change so that VideoSendStream::Start adds the stream as a BitrateObserver and VideoSendStream::Stop removes the stream as observer.
That also means that start will trigger a VideoEncoder::SetRate call with the most recent bitrate estimate.
VideoSendStream::Stop will trigger a VideoEncoder::SetRate with bitrate = 0.
BUG=webrtc:5687 b/28636240
Review-Url: https://codereview.webrtc.org/2070343002
Cr-Commit-Position: refs/heads/master@{#13192}