Refactor registerable callbacks for VideoBitrateObserver from rtp_rtcp module into vie_channel.

R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/20879004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6626 4adac7df-926f-26a2-2b94-8c16560cd09d