Add a pure virtual method to the GarbageCollectedMixin class

Source/modules/screen_orientation/ScreenOrientationDispatcher.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 ScreenOrientationDispatcher_h
 #define ScreenOrientationDispatcher_h
 
 #include "core/frame/PlatformEventDispatcher.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 // ScreenOrientationDispatcher is a singleton that handles whether the current
 // Blink instance should be listening to the screen orientation platform events.
 // It is not a common implementation of PlatformEventDispatcher in the sense
 // that it doesn't actually dispatch events but simply start/stop listening. The
 // reason being that screen orientation events are always sent to the WebFrame's
 // but some platforms require to poll to have an accurate reporting. When
 // ScreenOrientationDispatcher is listening, that means that the platform should
 // be polling if required.
 class ScreenOrientationDispatcher final : public GarbageCollectedFinalized<ScreenOrientationDispatcher>, public PlatformEventDispatcher {
+    USING_GARBAGE_COLLECTED_MIXIN(ScreenOrientationDispatcher);
 public:
     static ScreenOrientationDispatcher& instance();
 
     virtual void trace(Visitor*) override;
 
 private:
     ScreenOrientationDispatcher();
 
     // Inherited from PlatformEventDispatcher.
     virtual void startListening() override;
     virtual void stopListening() override;
 };
 
 } // namespace blink
 
 #endif // ScreenOrientationDispatcher_h