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

Source/core/events/OverflowEvent.cpp

Index: Source/core/events/OverflowEvent.cpp
diff --git a/Source/core/events/OverflowEvent.cpp b/Source/core/events/OverflowEvent.cpp
index 376404d7aa172f1340313964812fc634fbc81b08..a674e65481f6c66fdf92c7b5e4066683e2ed7dcc 100644
--- a/Source/core/events/OverflowEvent.cpp
+++ b/Source/core/events/OverflowEvent.cpp
@@ -76,4 +76,9 @@ const AtomicString& OverflowEvent::interfaceName() const
     return EventNames::OverflowEvent;
 }
 
+void OverflowEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 }