StreamStatisticianImpl.ssrc_ is protected by stream_lock_, should be cached while holding lock to avoid race condition. Also, rtp_callback_ do not need to be called in GetStatistics() at all BUG=2853 R=pbos@webrtc.org, stefan@webrtc.org Review URL: https://webrtc-codereview.appspot.com/7619004 git-svn-id: http://webrtc.googlecode.com/svn/trunk@5435 4adac7df-926f-26a2-2b94-8c16560cd09d