Also refactor TransportFeedback to use this. BUG= Review URL: https://codereview.webrtc.org/1307663004 Cr-Commit-Position: refs/heads/master@{#9935}
R=mflodman@webrtc.org, stefan@webrtc.org Review URL: https://webrtc-codereview.appspot.com/27799004 git-svn-id: http://webrtc.googlecode.com/svn/trunk@7559 4adac7df-926f-26a2-2b94-8c16560cd09d