Protecting Bitrate to avoid data race found by tsan.

TEST=try and vie_auto_test with tsan.
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2163004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4673 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed