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

Source/core/rendering/RenderIFrame.h

 /*
  * Copyright (C) 2010 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
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE COMPUTER, INC. ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE COMPUTER, INC. OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
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 #ifndef RenderIFrame_h
 #define RenderIFrame_h
 
-#include "RenderFrameBase.h"
+#include "RenderPart.h"
 
 namespace WebCore {
 
 class RenderView;
 
-class RenderIFrame : public RenderFrameBase {
+class RenderIFrame : public RenderPart {
 public:
     explicit RenderIFrame(Element*);
 
-    bool flattenFrame() const;
     bool isSeamless() const;
 
 private:
     virtual LayoutUnit minPreferredLogicalWidth() const OVERRIDE;
     virtual LayoutUnit maxPreferredLogicalWidth() const OVERRIDE;
 
     virtual bool shouldComputeSizeAsReplaced() const OVERRIDE;
     virtual bool isInlineBlockOrInlineTable() const OVERRIDE;
 
     virtual void layout() OVERRIDE;
@@ skipping 21 matching lines @@
     return static_cast<const RenderIFrame*>(object);
 }
 
 // This will catch anyone doing an unnecessary cast.
 void toRenderIFrame(const RenderIFrame*);
 
 
 } // namespace WebCore
 
 #endif // RenderIFrame_h