Implements TODOs for webrtc::datachannel state management when the SCTP association is congested.  Adds missing state variables for each step in the transitions between DataChannelInterface::DataStates (kConnecting, kOpen, etc.), and uses them.

BUG=https://code.google.com/p/chromium/issues/detail?id=474650
R=jiayl@webrtc.org, pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44299004

Cr-Commit-Position: refs/heads/master@{#9331}
4 files changed