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

Source/core/events/WheelEvent.cpp

Index: Source/core/events/WheelEvent.cpp
diff --git a/Source/core/events/WheelEvent.cpp b/Source/core/events/WheelEvent.cpp
index 80accef7d6b0054d78f30bd3dc0ab3a0835af8b6..74954307e8e25f609491fa0de4508c4ce8e23f69 100644
--- a/Source/core/events/WheelEvent.cpp
+++ b/Source/core/events/WheelEvent.cpp
@@ -126,6 +126,11 @@ bool WheelEvent::isWheelEvent() const
     return true;
 }
 
+void WheelEvent::trace(Visitor* visitor)
+{
+    MouseEvent::trace(visitor);
+}
+
 inline static unsigned deltaMode(const PlatformWheelEvent& event)
 {
     return event.granularity() == ScrollByPageWheelEvent ? WheelEvent::DOM_DELTA_PAGE : WheelEvent::DOM_DELTA_PIXEL;