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

Source/core/storage/StorageEvent.h

Index: Source/core/storage/StorageEvent.h
diff --git a/Source/core/storage/StorageEvent.h b/Source/core/storage/StorageEvent.h
index dfe6b8a00627c2e949bccd16843ae43625908a7a..661456e2272a24979569ac7f59a12522e3d2ce89 100644
--- a/Source/core/storage/StorageEvent.h
+++ b/Source/core/storage/StorageEvent.h
@@ -64,6 +64,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     StorageEvent();
     StorageEvent(const AtomicString& type, const String& key, const String& oldValue, const String& newValue, const String& url, Storage* storageArea);