Fix virtual/override/final usage in the rest of Source/core/.

Source/core/frame/PlatformEventController.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 PlatformEventController_h
 #define PlatformEventController_h
 
 #include "core/CoreExport.h"
 #include "core/page/PageLifecycleObserver.h"
 #include "platform/Timer.h"
@@ skipping 20 matching lines @@
     virtual void registerWithDispatcher() = 0;
     virtual void unregisterWithDispatcher() = 0;
 
     // When true initiates a one-shot didUpdateData() when startUpdating() is called.
     virtual bool hasLastData() = 0;
 
     bool m_hasEventListener;
 
 private:
     // Inherited from PageLifecycleObserver.
-    virtual void pageVisibilityChanged() override;
+    void pageVisibilityChanged() override;
 
     void oneShotCallback(Timer<PlatformEventController>*);
 
     bool m_isActive;
     Timer<PlatformEventController> m_timer;
 };
 
 } // namespace blink
 
 #endif // PlatformEventController_h