Clean up forward declarations in Source/platform

Source/platform/LengthBox.h

 /*
     Copyright (C) 1999 Lars Knoll (knoll@kde.org)
     Copyright (C) 2006, 2008 Apple Inc. All rights reserved.
     Copyright (c) 2012, Google Inc. All rights reserved.
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later version.
 
@@ skipping 11 matching lines @@
 #ifndef LengthBox_h
 #define LengthBox_h
 
 #include "platform/Length.h"
 #include "platform/PlatformExport.h"
 #include "platform/text/TextDirection.h"
 #include "platform/text/WritingMode.h"
 
 namespace blink {
 
-class RenderStyle;
-
 class PLATFORM_EXPORT LengthBox {
 public:
     LengthBox()
     {
     }
 
     LengthBox(LengthType t)
         : m_left(t)
         , m_right(t)
         , m_top(t)
@@ skipping 56 matching lines @@
     // Must be public for SET_VAR in RenderStyle.h
     Length m_left;
     Length m_right;
     Length m_top;
     Length m_bottom;
 };
 
 } // namespace blink
 
 #endif // LengthBox_h