Lock RTPSender statistics.

Suppressing these errors in TSan has become tedious. It's better to just
lock them.

BUG=2349
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2197004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@4713 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed