Suppress webrtc trace races detected by tsan.

BUG=3372
R=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17579004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6226 4adac7df-926f-26a2-2b94-8c16560cd09d