Oilpan: fix build after r177520.

Source/modules/serviceworkers/NavigatorServiceWorker.cpp

 // 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.
 
 #include "config.h"
 #include "modules/serviceworkers/NavigatorServiceWorker.h"
 
 #include "core/dom/Document.h"
 #include "core/frame/LocalDOMWindow.h"
 #include "core/frame/LocalFrame.h"
@@ skipping 56 matching lines @@
 }
 
 void NavigatorServiceWorker::willDetachGlobalObjectFromFrame()
 {
     if (m_serviceWorker) {
         m_serviceWorker->detachClient();
         m_serviceWorker = nullptr;
     }
 }
 
+void NavigatorServiceWorker::trace(Visitor* visitor)
+{
+    visitor->trace(m_serviceWorker);
+    WillBeHeapSupplement<Navigator>::trace(visitor);
+}
+
 } // namespace WebCore