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

Source/modules/indexeddb/IDBVersionChangeEvent.h

Index: Source/modules/indexeddb/IDBVersionChangeEvent.h
diff --git a/Source/modules/indexeddb/IDBVersionChangeEvent.h b/Source/modules/indexeddb/IDBVersionChangeEvent.h
index f197010525a3c18737e405d73bef8dae3537b01f..d5ceab1683368be6cb7a4a1aadf0f2716b4a1e2e 100644
--- a/Source/modules/indexeddb/IDBVersionChangeEvent.h
+++ b/Source/modules/indexeddb/IDBVersionChangeEvent.h
@@ -48,6 +48,8 @@ public:
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     IDBVersionChangeEvent(PassRefPtr<IDBAny> oldVersion, PassRefPtr<IDBAny> newVersion, const AtomicString& eventType, blink::WebIDBDataLoss, const String& dataLoss);