Fix a crash in statscollector.cc caused by invoking methods on the worker thread which destroys the Transport.

BUG=3579
R=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17999004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6776 4adac7df-926f-26a2-2b94-8c16560cd09d