Make setting identical RTP extensions a no-op.

Setting extensions are responsible for a lot of stream tear-downs
causing substantial slowdowns in SetRemoteDescription.

BUG=1788,4077
R=pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/40389004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7998 4adac7df-926f-26a2-2b94-8c16560cd09d
3 files changed