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

Source/modules/indexeddb/IDBVersionChangeEvent.cpp

Index: Source/modules/indexeddb/IDBVersionChangeEvent.cpp
diff --git a/Source/modules/indexeddb/IDBVersionChangeEvent.cpp b/Source/modules/indexeddb/IDBVersionChangeEvent.cpp
index 798b9ac4c4085a7fa209cdd74681dca24540fe2a..13a89c095d5f474a87cafbe177bcc7d692b6a36c 100644
--- a/Source/modules/indexeddb/IDBVersionChangeEvent.cpp
+++ b/Source/modules/indexeddb/IDBVersionChangeEvent.cpp
@@ -76,4 +76,9 @@ const AtomicString& IDBVersionChangeEvent::interfaceName() const
     return EventNames::IDBVersionChangeEvent;
 }
 
+void IDBVersionChangeEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 } // namespace WebCore