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

Source/core/events/MouseEvent.h

Index: Source/core/events/MouseEvent.h
diff --git a/Source/core/events/MouseEvent.h b/Source/core/events/MouseEvent.h
index 8b17aab8efee8c926b03cdd60f1b2f690c70c308..77ba12b5925633c13c7db131ecd8ba19c43bcbe6 100644
--- a/Source/core/events/MouseEvent.h
+++ b/Source/core/events/MouseEvent.h
@@ -102,11 +102,13 @@ protected:
 
     MouseEvent();
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     unsigned short m_button;
     bool m_buttonDown;
     RefPtr<EventTarget> m_relatedTarget;
-    RefPtrWillBePersistent<Clipboard> m_clipboard;
+    RefPtrWillBeMember<Clipboard> m_clipboard;
 };
 
 class SimulatedMouseEvent FINAL : public MouseEvent {
@@ -114,6 +116,8 @@ public:
     static PassRefPtr<SimulatedMouseEvent> create(const AtomicString& eventType, PassRefPtr<AbstractView>, PassRefPtr<Event> underlyingEvent);
     virtual ~SimulatedMouseEvent();
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     SimulatedMouseEvent(const AtomicString& eventType, PassRefPtr<AbstractView>, PassRefPtr<Event> underlyingEvent);
 };