Clean up forward declarations in Source/platform

Source/platform/scroll/ScrollableArea.h

 /*
  * Copyright (C) 2008, 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 17 matching lines @@
 
 #include "platform/PlatformExport.h"
 #include "platform/scroll/ScrollAnimator.h"
 #include "platform/scroll/Scrollbar.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/Vector.h"
 
 namespace blink {
 
 class FloatPoint;
-class GraphicsContext;
 class GraphicsLayer;
 class HostWindow;
-class PlatformGestureEvent;
 class PlatformWheelEvent;
 class ProgrammaticScrollAnimator;
 class ScrollAnimator;
 
 enum ScrollBehavior {
     ScrollBehaviorAuto,
     ScrollBehaviorInstant,
     ScrollBehaviorSmooth,
 };
 
@@ skipping 244 matching lines @@
     // vertical-lr / ltr            NO                      NO
     // vertical-lr / rtl            NO                      YES
     // vertical-rl / ltr            YES                     NO
     // vertical-rl / rtl            YES                     YES
     IntPoint m_scrollOrigin;
 };
 
 } // namespace blink
 
 #endif // ScrollableArea_h