Always give RenderParts a RenderLayer.

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 3581 matching lines @@
         childLocalRect.moveBy(-childOffset);
 
         if (childLayer->backgroundIsKnownToBeOpaqueInRect(childLocalRect))
             return true;
     }
     return false;
 }
 
 bool RenderLayer::shouldBeSelfPaintingLayer() const
 {
+    if (renderer()->isRenderPart() && toRenderPart(renderer())->requiresAcceleratedCompositing())
+        return true;
     return m_layerType == NormalLayer
         || (m_scrollableArea && m_scrollableArea->hasOverlayScrollbars())
         || needsCompositedScrolling();
 }
 
 void RenderLayer::updateSelfPaintingLayer()
 {
     bool isSelfPaintingLayer = this->shouldBeSelfPaintingLayer();
     if (this->isSelfPaintingLayer() == isSelfPaintingLayer)
         return;
@@ skipping 335 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif