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

Source/core/rendering/RenderPart.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Simon Hausmann <hausmann@kde.org>
  * Copyright (C) 2006, 2009 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.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  *
  */
 
 #ifndef RenderPart_h
 #define RenderPart_h
 
 #include "RenderWidget.h"
 
 namespace WebCore {
 
-// Renderer for frames via RenderFrameBase, and plug-ins via RenderEmbeddedObject.
+// Renderer for frames and plug-ins, the latter via RenderEmbeddedObject.
 class RenderPart : public RenderWidget {
 public:
     explicit RenderPart(Element*);
     virtual ~RenderPart();
 
     virtual void setWidget(PassRefPtr<Widget>);
     virtual void viewCleared();
 
     bool requiresAcceleratedCompositing() const;
 
@@ skipping 15 matching lines @@
     ASSERT_WITH_SECURITY_IMPLICATION(!object || object->isRenderPart());
     return static_cast<RenderPart*>(object);
 }
 
 // This will catch anyone doing an unnecessary cast.
 void toRenderPart(const RenderPart*);
 
 }
 
 #endif