talk_base: remove lock inversion between MessageQueue and MessageQueueManager.

Removes the concept of a MessageQueue being "active" in favor of considering all
live MQ's to be active.
(previously a MQ was active starting from the first Post to it and stopped being
active in its dtor).

BUG=3230
R=sriniv@google.com

Review URL: https://webrtc-codereview.appspot.com/21489004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6190 4adac7df-926f-26a2-2b94-8c16560cd09d