Implements start bitrate for new video API.

Added  a new rampup test.

BUG=2879
R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15769004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6443 4adac7df-926f-26a2-2b94-8c16560cd09d