Refactor root element background painting

Source/core/paint/ViewPainter.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 ViewPainter_h
 #define ViewPainter_h
 
 namespace blink {
 
 class LayoutPoint;
 struct PaintInfo;
 class LayoutBox;
 class LayoutView;
 
 class ViewPainter {
 public:
     ViewPainter(LayoutView& layoutView) : m_layoutView(layoutView) { }
 
     void paint(const PaintInfo&, const LayoutPoint& paintOffset);
     void paintBoxDecorationBackground(const PaintInfo&);
 
 private:
-    bool rootFillsViewportBackground(LayoutBox* rootBox) const;
-
     LayoutView& m_layoutView;
 };
 
 } // namespace blink
 
 #endif // ViewPainter_h