Oilpan: fix build after r187279.

Source/core/storage/DOMWindowStorageController.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 DOMWindowStorageController_h
 #define DOMWindowStorageController_h
 
 #include "core/frame/DOMWindowLifecycleObserver.h"
 #include "platform/Supplementable.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class Document;
 class Event;
 
-class DOMWindowStorageController : public NoBaseWillBeGarbageCollectedFinalized<DOMWindowStorageController>, public WillBeHeapSupplement<Document>, public DOMWindowLifecycleObserver {
+class DOMWindowStorageController final : public NoBaseWillBeGarbageCollectedFinalized<DOMWindowStorageController>, public WillBeHeapSupplement<Document>, public DOMWindowLifecycleObserver {
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(DOMWindowStorageController);
 public:
     virtual ~DOMWindowStorageController();
+    virtual void trace(Visitor*);
 
     static const char* supplementName();
     static DOMWindowStorageController& from(Document&);
 
     // Inherited from DOMWindowLifecycleObserver
     virtual void didAddEventListener(LocalDOMWindow*, const AtomicString&) override;
 
 protected:
     explicit DOMWindowStorageController(Document&);
 
 private:
     Document& document() const { return *m_document; }
 
     RawPtrWillBeMember<Document> m_document;
 };
 
 } // namespace blink
 
 #endif // DOMWindowStorageController_h