Add a pure virtual method to the GarbageCollectedMixin class

Source/modules/presentation/Presentation.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 Presentation_h
 #define Presentation_h
 
 #include "core/dom/ContextLifecycleObserver.h"
 #include "core/events/EventTarget.h"
 
 namespace blink {
 
 class Presentation final
     : public RefCountedGarbageCollectedWillBeGarbageCollectedFinalized<Presentation>
     , public EventTargetWithInlineData
     , public ContextLifecycleObserver {
     DEFINE_EVENT_TARGET_REFCOUNTING_WILL_BE_REMOVED(RefCountedGarbageCollected<Presentation>);
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(Presentation);
     DEFINE_WRAPPERTYPEINFO();
 public:
     static Presentation* create(ExecutionContext*);
     virtual ~Presentation();
 
     // EventTarget implementation.
     virtual const AtomicString& interfaceName() const override;
     virtual ExecutionContext* executionContext() const override;
 
     virtual void trace(Visitor*) override;
 
     DEFINE_ATTRIBUTE_EVENT_LISTENER(availablechange);
 
 private:
     explicit Presentation(ExecutionContext*);
 };
 
 } // namespace blink
 
 #endif // Presentation_h