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

Source/core/events/FocusEvent.h

Index: Source/core/events/FocusEvent.h
diff --git a/Source/core/events/FocusEvent.h b/Source/core/events/FocusEvent.h
index 517f71f3f4cc4c541911b5211fb2e9b4d46707fb..fd66f186ec78a96005a68975858db3f7a94e0436 100644
--- a/Source/core/events/FocusEvent.h
+++ b/Source/core/events/FocusEvent.h
@@ -63,6 +63,8 @@ public:
     virtual const AtomicString& interfaceName() const OVERRIDE;
     virtual bool isFocusEvent() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     FocusEvent();
     FocusEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtr<AbstractView>, int, EventTarget*);