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

Source/modules/mediastream/MediaStreamTrackEvent.h

Index: Source/modules/mediastream/MediaStreamTrackEvent.h
diff --git a/Source/modules/mediastream/MediaStreamTrackEvent.h b/Source/modules/mediastream/MediaStreamTrackEvent.h
index eda894a016b2041e7ee73aee469e36d6daf931d6..def39df7f8db1cddc6c78f2b2be50f9bc44a66df 100644
--- a/Source/modules/mediastream/MediaStreamTrackEvent.h
+++ b/Source/modules/mediastream/MediaStreamTrackEvent.h
@@ -44,6 +44,8 @@ public:
     // Event
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     MediaStreamTrackEvent();
     MediaStreamTrackEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtr<MediaStreamTrack>);