Also fixes data races in RTCPSender::SetCSRCStatus() and RTCPSender::SetStartTimestamp(). BUG= R=tommi@webrtc.org Review URL: https://webrtc-codereview.appspot.com/20919004 git-svn-id: http://webrtc.googlecode.com/svn/trunk@6666 4adac7df-926f-26a2-2b94-8c16560cd09d