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both TransmitMixer::PrepareDemux() and StartSend()/StopSend(). Put a lock to resolve it. Review URL: http://webrtc-codereview.appspot.com/293005 git-svn-id: http://webrtc.googlecode.com/svn/trunk@1038 4adac7df-926f-26a2-2b94-8c16560cd09d