MediaStream API: Making WebRTCDataChannelHandler pure virtual again
Chromium has implemented these new functions so I'm cleaning up the API.

Review URL: https://chromiumcodereview.appspot.com/23600022

git-svn-id: svn://svn.chromium.org/blink/trunk@157519 bbb929c8-8fbe-4397-9dbb-9b2b20218538