Expose RTCDataChannel and RTCDataChannelEvent (with constructor)

third_party/WebKit/Source/modules/peerconnection/RTCDataChannelEvent.idl

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
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 // https://w3c.github.io/webrtc-pc/#rtcdatachannelevent
 
-// TODO(foolip): RTCDataChannelEvent should have a constructor. See also
-// https://github.com/w3c/webrtc-pc/issues/817
 [
-    NoInterfaceObject
+    Constructor(DOMString type, RTCDataChannelEventInit eventInitDict),
 ] interface RTCDataChannelEvent : Event {
     readonly attribute RTCDataChannel channel;
 };