MediaStream API: Deleting RTCDataChannelHandler
Removing the unnecessary bridging class RTCDataChannelHandler as part of
the effort to get rid of Source/core/platform/mediastream/

Review URL: https://codereview.chromium.org/143523008

git-svn-id: svn://svn.chromium.org/blink/trunk@165516 bbb929c8-8fbe-4397-9dbb-9b2b20218538