Do not assert for blocking call allowed in Thread::Join.
We do not allow blocking call from the worker thread, but on Android the worker thread may stop/join a SignalThread, which hits the assert.
AssertBlockingIsAllowedOnCurrentThread is used to make sure a thread does not do Invoke, so check that in Thread::Join does not seem to add much value.

BUG=3857
R=juberti@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/24709004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7308 4adac7df-926f-26a2-2b94-8c16560cd09d