Oilpan: fix build after r368814.

third_party/WebKit/Source/core/dom/IntersectionObserverController.h

 // Copyright 2016 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 IntersectionObserverController_h
 #define IntersectionObserverController_h
 
 #include "core/dom/Element.h"
 #include "core/dom/IntersectionObserver.h"
 #include "platform/Timer.h"
 #include "platform/heap/Handle.h"
 #include "wtf/HashSet.h"
 
 // Design doc for IntersectionObserver implementation:
 //   https://docs.google.com/a/google.com/document/d/1hLK0eyT5_BzyNS4OkjsnoqqFQDYCbKfyBinj94OnLiQ
 
 namespace blink {
 
 class IntersectionObserverController : public GarbageCollectedFinalized<IntersectionObserverController>, public ActiveDOMObject {
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(IntersectionObserverController);
 public:
     static IntersectionObserverController* create(Document*);
     ~IntersectionObserverController();
 
     void resume() override;
 
     void scheduleIntersectionObserverForDelivery(IntersectionObserver&);
     void deliverIntersectionObservations(Timer<IntersectionObserverController>*);
     void computeTrackedIntersectionObservations();
     void addTrackedObserver(IntersectionObserver&);
@@ skipping 10 matching lines @@
     HeapHashSet<WeakMember<IntersectionObserver>> m_trackedIntersectionObservers;
     // IntersectionObservers for which this is the execution context of the callback.
     HeapHashSet<Member<IntersectionObserver>> m_pendingIntersectionObservers;
 
     bool m_timerFiredWhileSuspended;
 };
 
 } // namespace blink
 
 #endif // IntersectionObserverController_h