Protect write of send_target_bitrate.

This issue was catch by tsan bot.

BUG=3065
R=stefan@webrtc.org, andrew

Review URL: https://webrtc-codereview.appspot.com/10619004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5790 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed