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

Source/core/events/TransitionEvent.h

Index: Source/core/events/TransitionEvent.h
diff --git a/Source/core/events/TransitionEvent.h b/Source/core/events/TransitionEvent.h
index 91bab0e3dbc3a72aea6cd862bf4707b2892d2a57..27d774a71a4a4a5481102960d7ed2b9838e7c28a 100644
--- a/Source/core/events/TransitionEvent.h
+++ b/Source/core/events/TransitionEvent.h
@@ -62,6 +62,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     TransitionEvent();
     TransitionEvent(const AtomicString& type, const String& propertyName, double elapsedTime, const String& pseudoElement);