Bug 1447692: update RTCDataChannelEventInit to meet spec. r=smaug

MozReview-Commit-ID: 9ggm11SsOyC

--HG--
extra : rebase_source : c50701eddb12de3a86a128662f9671cd31186f57

dom/webidl/RTCDataChannelEvent.webidl

@@ -8,11 +8,11 @@
  */
 
 dictionary RTCDataChannelEventInit : EventInit {
-    RTCDataChannel? channel = null;
+    required RTCDataChannel channel;
 };
 
 [Pref="media.peerconnection.enabled",
- Constructor(DOMString type, optional RTCDataChannelEventInit eventInitDict)]
+ Constructor(DOMString type, RTCDataChannelEventInit eventInitDict)]
 interface RTCDataChannelEvent : Event {
-  readonly attribute RTCDataChannel? channel;
+  readonly attribute RTCDataChannel channel;
 };