Make Event RefCountedGarbageCollected and implement trace() methods to the Event hierarcy

Source/modules/mediastream/RTCDataChannelEvent.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 48 matching lines @@
 RTCDataChannel* RTCDataChannelEvent::channel() const
 {
     return m_channel.get();
 }
 
 const AtomicString& RTCDataChannelEvent::interfaceName() const
 {
     return EventNames::RTCDataChannelEvent;
 }
 
+void RTCDataChannelEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 } // namespace WebCore
-