Always give RenderParts a RenderLayer.

Source/core/rendering/RenderIFrame.cpp

 /*
  * 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
@@ skipping 35 matching lines @@
     return true;
 }
 
 bool RenderIFrame::isInlineBlockOrInlineTable() const
 {
     return isInline();
 }
 
 LayerType RenderIFrame::layerTypeRequired() const
 {
-    LayerType type = RenderPart::layerTypeRequired();
-    if (type != NoLayer)
-        return type;
-
     if (style()->resize() != RESIZE_NONE)
         return NormalLayer;
-
-    return ForcedLayer;
+    return RenderPart::layerTypeRequired();
 }
 
 void RenderIFrame::layout()
 {
     ASSERT(needsLayout());
 
     updateLogicalWidth();
     // No kids to layout as a replaced element.
     updateLogicalHeight();
 
     m_overflow.clear();
     addVisualEffectOverflow();
     updateLayerTransform();
 
     clearNeedsLayout();
 }
 
 }