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

Source/core/events/MouseRelatedEvent.h

Index: Source/core/events/MouseRelatedEvent.h
diff --git a/Source/core/events/MouseRelatedEvent.h b/Source/core/events/MouseRelatedEvent.h
index ee70b84087caf363bdcbe9935b5deae1131401ad..bc6863cf208cac4d663ff71f2218cbc1866100a5 100644
--- a/Source/core/events/MouseRelatedEvent.h
+++ b/Source/core/events/MouseRelatedEvent.h
@@ -57,6 +57,8 @@ namespace WebCore {
         const LayoutPoint& absoluteLocation() const { return m_absoluteLocation; }
         void setAbsoluteLocation(const LayoutPoint& p) { m_absoluteLocation = p; }
 
+        virtual void trace(Visitor*) OVERRIDE;
+
     protected:
         MouseRelatedEvent();
         MouseRelatedEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtr<AbstractView>,