Don't always fully repaint on viewport resize

Source/core/rendering/RenderView.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2006 Apple Computer, Inc.
  *
  * 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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 224 matching lines @@
 
     void pushLayoutStateForCurrentFlowThread(const RenderObject&);
     void popLayoutStateForCurrentFlowThread();
 
     friend class LayoutStateMaintainer;
     friend class LayoutStateDisabler;
     friend class RootLayoutStateScope;
 
     bool shouldUsePrintingLayout() const;
 
+    RenderObject* backgroundRenderer() const;
+
     FrameView* m_frameView;
 
     RenderObject* m_selectionStart;
     RenderObject* m_selectionEnd;
 
     int m_selectionStartPos;
     int m_selectionEndPos;
 
     LayoutUnit m_pageLogicalHeight;
     bool m_pageLogicalHeightChanged;
@@ skipping 119 matching lines @@
     {
         m_view.enableLayoutState();
     }
 private:
     RenderView& m_view;
 };
 
 } // namespace WebCore
 
 #endif // RenderView_h