Oilpan: fix build after r179903.

Source/modules/serviceworkers/ServiceWorkerRegistration.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ServiceWorkerRegistration_h
 #define ServiceWorkerRegistration_h
 
 #include "core/dom/ActiveDOMObject.h"
 #include "core/events/EventTarget.h"
 #include "modules/serviceworkers/ServiceWorker.h"
@@ skipping 39 matching lines @@
     PassRefPtrWillBeRawPtr<ServiceWorker> installing() { return m_installing.get(); }
     PassRefPtrWillBeRawPtr<ServiceWorker> waiting() { return m_waiting.get(); }
     PassRefPtrWillBeRawPtr<ServiceWorker> active() { return m_active.get(); }
 
     String scope() const;
 
     ScriptPromise unregister(ScriptState*);
 
     DEFINE_ATTRIBUTE_EVENT_LISTENER(updatefound);
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     static PassRefPtrWillBeRawPtr<ServiceWorkerRegistration> create(ExecutionContext*, PassOwnPtr<WebServiceWorkerRegistration>);
     ServiceWorkerRegistration(ExecutionContext*, PassOwnPtr<WebServiceWorkerRegistration>);
 
     OwnPtr<WebServiceWorkerRegistration> m_outerRegistration;
     WebServiceWorkerProvider* m_provider;
     RefPtrWillBeMember<ServiceWorker> m_installing;
     RefPtrWillBeMember<ServiceWorker> m_waiting;
     RefPtrWillBeMember<ServiceWorker> m_active;
 };
 
 } // namespace blink
 
 #endif // ServiceWorkerRegistration_h