Fix deadlock on register/unregister observer while there is a an going callback.

BUG=2835
R=mallinath@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/7119005

git-svn-id: http://webrtc.googlecode.com/svn/trunk/talk@5421 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed