Drop the DataChannel message if it's received when the channel is not open.
It may happen when the JS has closed the channel on the signaling thread while messages are received on the worker thread and posted before the state change is pushed to the worker thread.

BUG=crbug/363005
R=mallinath@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/19469005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6161 4adac7df-926f-26a2-2b94-8c16560cd09d