Revert r6161 "Drop the DataChannel message if it's received when the channel is not open."
The spec does not say the DataChannel has to be open to receive a message.

TBR=pthatcher@google.com
BUG=crbug/363005

Review URL: https://webrtc-codereview.appspot.com/16569004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6264 4adac7df-926f-26a2-2b94-8c16560cd09d