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

Source/modules/mediastream/MediaStreamEvent.h

Index: Source/modules/mediastream/MediaStreamEvent.h
diff --git a/Source/modules/mediastream/MediaStreamEvent.h b/Source/modules/mediastream/MediaStreamEvent.h
index 6eae92b0247176b81786b48ce1a9846c77e8e7c3..fc646fb3e2d0afed0cb4ecec759d5b467fb555ee 100644
--- a/Source/modules/mediastream/MediaStreamEvent.h
+++ b/Source/modules/mediastream/MediaStreamEvent.h
@@ -50,6 +50,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     MediaStreamEvent();
     MediaStreamEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtr<MediaStream>);