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

Source/modules/mediastream/RTCDataChannelEvent.h

Index: Source/modules/mediastream/RTCDataChannelEvent.h
diff --git a/Source/modules/mediastream/RTCDataChannelEvent.h b/Source/modules/mediastream/RTCDataChannelEvent.h
index ce5fe3c81aa9777bc38bd8ed08b051b3e87f1fea..f34d1c4868cb2048093dc1223b33120f3eb476eb 100644
--- a/Source/modules/mediastream/RTCDataChannelEvent.h
+++ b/Source/modules/mediastream/RTCDataChannelEvent.h
@@ -42,6 +42,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     RTCDataChannelEvent();
     RTCDataChannelEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtr<RTCDataChannel>);