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

Source/core/events/MouseRelatedEvent.cpp

Index: Source/core/events/MouseRelatedEvent.cpp
diff --git a/Source/core/events/MouseRelatedEvent.cpp b/Source/core/events/MouseRelatedEvent.cpp
index cc4e9026a9bcf630fe2e22cbce263dc641c23321..6918ae3d6283ffc6e043badd52c3ec0fb2291ebc 100644
--- a/Source/core/events/MouseRelatedEvent.cpp
+++ b/Source/core/events/MouseRelatedEvent.cpp
@@ -222,4 +222,9 @@ int MouseRelatedEvent::y() const
     return m_clientLocation.y();
 }
 
+void MouseRelatedEvent::trace(Visitor* visitor)
+{
+    UIEventWithKeyState::trace(visitor);
+}
+
 } // namespace WebCore