Adding a payload type for RTX.

BUG=736
TEST=Modified RTP unittests.

Review URL: https://webrtc-codereview.appspot.com/1278004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@3843 4adac7df-926f-26a2-2b94-8c16560cd09d
17 files changed