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

Source/core/events/WebKitAnimationEvent.h

Index: Source/core/events/WebKitAnimationEvent.h
diff --git a/Source/core/events/WebKitAnimationEvent.h b/Source/core/events/WebKitAnimationEvent.h
index 731f45d716e5e1eafeaf65f22fe3fec5b192f0ae..0c4549ab289bc5cf2e25d076b357a0387cc97e55 100644
--- a/Source/core/events/WebKitAnimationEvent.h
+++ b/Source/core/events/WebKitAnimationEvent.h
@@ -62,6 +62,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     WebKitAnimationEvent();
     WebKitAnimationEvent(const AtomicString& type, const String& animationName, double elapsedTime);