Improve Call tests for RTX.

Also does some refactoring to reuse RtpRtcpObserver.

BUG=1811
R=mflodman@webrtc.org, pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/3809004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5126 4adac7df-926f-26a2-2b94-8c16560cd09d
6 files changed