Remove frame flattening support as Chromium has no intention of using it

Source/core/rendering/RenderFrameSet.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Simon Hausmann <hausmann@kde.org>
  * Copyright (C) 2006, 2008 Apple 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 92 matching lines @@
     virtual const char* renderName() const { return "RenderFrameSet"; }
     virtual bool isFrameSet() const { return true; }
 
     virtual void layout();
     virtual void paint(PaintInfo&, const LayoutPoint&);
     virtual bool isChildAllowed(RenderObject*, RenderStyle*) const;
     virtual CursorDirective getCursor(const LayoutPoint&, Cursor&) const;
 
     inline HTMLFrameSetElement* frameSet() const;
 
-    bool flattenFrameSet() const;
-
     void setIsResizing(bool);
 
     void layOutAxis(GridAxis&, const Length*, int availableSpace);
     void computeEdgeInfo();
     void fillFromEdgeInfo(const FrameEdgeInfo& edgeInfo, int r, int c);
     void positionFrames();
-    void positionFramesWithFlattening();
 
     int splitPosition(const GridAxis&, int split) const;
     int hitTestSplit(const GridAxis&, int position) const;
 
     void startResizing(GridAxis&, int position);
     void continueResizing(GridAxis&, int position);
 
     void paintRowBorder(const PaintInfo&, const IntRect&);
     void paintColumnBorder(const PaintInfo&, const IntRect&);
 
@@ skipping 12 matching lines @@
     ASSERT_WITH_SECURITY_IMPLICATION(!object || object->isFrameSet());
     return static_cast<RenderFrameSet*>(object);
 }
 
 // This will catch anyone doing an unnecessary cast.
 void toRenderFrameSet(const RenderFrameSet*);
 
 } // namespace WebCore
 
 #endif // RenderFrameSet_h