Possibly fix deadlock happening due to unregister/register modules as switching between AST and TSO estimators.

I think this does not introduces any contention or new deadlocks. But that is hard to verify at the moment.

BUG=chromium:388191
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17869004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6582 4adac7df-926f-26a2-2b94-8c16560cd09d