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

Source/modules/webaudio/OfflineAudioCompletionEvent.cpp

Index: Source/modules/webaudio/OfflineAudioCompletionEvent.cpp
diff --git a/Source/modules/webaudio/OfflineAudioCompletionEvent.cpp b/Source/modules/webaudio/OfflineAudioCompletionEvent.cpp
index 45f73e94e73a88a055d47a88418447ebb49ae6e9..19097a288af11eacab281cc9465a4d1144629b59 100644
--- a/Source/modules/webaudio/OfflineAudioCompletionEvent.cpp
+++ b/Source/modules/webaudio/OfflineAudioCompletionEvent.cpp
@@ -63,6 +63,11 @@ const AtomicString& OfflineAudioCompletionEvent::interfaceName() const
     return EventNames::OfflineAudioCompletionEvent;
 }
 
+void OfflineAudioCompletionEvent::trace(Visitor* visitor)
+{
+    Event::trace(visitor);
+}
+
 } // namespace WebCore
 
 #endif // ENABLE(WEB_AUDIO)