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

Source/core/events/GestureEvent.h

Index: Source/core/events/GestureEvent.h
diff --git a/Source/core/events/GestureEvent.h b/Source/core/events/GestureEvent.h
index a9696d22d0bb0383647fc9379ff422ae8f65c268..b02ed4453fcc43ce8b00016116bdc3e16cadc01c 100644
--- a/Source/core/events/GestureEvent.h
+++ b/Source/core/events/GestureEvent.h
@@ -45,6 +45,8 @@ public:
     float deltaX() const { return m_deltaX; }
     float deltaY() const { return m_deltaY; }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     GestureEvent();
     GestureEvent(const AtomicString& type, PassRefPtr<AbstractView>, int screenX, int screenY, int clientX, int clientY, bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, float deltaX, float deltaY);