Always give RenderParts a RenderLayer.

Source/core/rendering/RenderPart.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Simon Hausmann <hausmann@kde.org>
  *           (C) 2000 Stefan Schimanski (1Stein@gmx.de)
  * Copyright (C) 2004, 2005, 2006, 2009 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. 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
@@ skipping 35 matching lines @@
 
 RenderPart::~RenderPart()
 {
 }
 
 LayerType RenderPart::layerTypeRequired() const
 {
     LayerType type = RenderWidget::layerTypeRequired();
     if (type != NoLayer)
         return type;
-
-    return requiresAcceleratedCompositing() ? NormalLayer : NoLayer;
+    return ForcedLayer;
 }
 
 bool RenderPart::requiresAcceleratedCompositing() const
 {
     // There are two general cases in which we can return true. First, if this is a plugin
     // renderer and the plugin has a layer, then we need a layer. Second, if this is
     // a renderer with a contentDocument and that document needs a layer, then we need
     // a layer.
     if (widget() && widget()->isPluginView() && toPluginView(widget())->platformLayer())
         return true;
@@ skipping 63 matching lines @@
 }
 
 CompositingReasons RenderPart::additionalCompositingReasons(CompositingTriggerFlags) const
 {
     if (requiresAcceleratedCompositing())
         return CompositingReasonIFrame;
     return CompositingReasonNone;
 }
 
 }