Oilpan: fix build after r179903.

Source/modules/serviceworkers/ServiceWorkerRegistration.h

Index: Source/modules/serviceworkers/ServiceWorkerRegistration.h
diff --git a/Source/modules/serviceworkers/ServiceWorkerRegistration.h b/Source/modules/serviceworkers/ServiceWorkerRegistration.h
index d5160fd18f3aa1a62b71d44b3555a90ddd933cc8..903657f8e76312335574948b2fb7b2c86b210243 100644
--- a/Source/modules/serviceworkers/ServiceWorkerRegistration.h
+++ b/Source/modules/serviceworkers/ServiceWorkerRegistration.h
@@ -57,6 +57,8 @@ public:
 
     DEFINE_ATTRIBUTE_EVENT_LISTENER(updatefound);
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     static PassRefPtrWillBeRawPtr<ServiceWorkerRegistration> create(ExecutionContext*, PassOwnPtr<WebServiceWorkerRegistration>);
     ServiceWorkerRegistration(ExecutionContext*, PassOwnPtr<WebServiceWorkerRegistration>);