Change BitrateStats to more generalized RateStatistics

BUG=2656
R=holmer@google.com, mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/4559004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5202 4adac7df-926f-26a2-2b94-8c16560cd09d
8 files changed