Revert of Revert of Put the squashing layer underneath the ancestor clipping layer if present.

Source/core/rendering/compositing/CompositedLayerMapping.h

 /*
  * Copyright (C) 2009, 2010, 2011 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 331 matching lines @@
     //
     // With the hierarchy set up like this, the root content layer is able to scroll without affecting
     // the background layer (or repainting).
     OwnPtr<GraphicsLayer> m_foregroundLayer; // Only used in cases where we need to draw the foreground separately.
     OwnPtr<GraphicsLayer> m_backgroundLayer; // Only used in cases where we need to draw the background separately.
 
     OwnPtr<GraphicsLayer> m_layerForHorizontalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForVerticalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForScrollCorner;
 
-    OwnPtr<GraphicsLayer> m_squashingContainmentLayer; // Only used if any squashed layers exist, to contain the squashed layers as siblings to the rest of the GraphicsLayer tree chunk.
+    // A squashing CLM has two possible squashing-related structures.
+    //
+    // If m_clippingAncestorLayer is present:
+    //
+    // m_clippingAncestorLayer
+    //   + m_graphicsLayer
+    //   + m_squashingLayer
+    //
+    // If not:
+    //
+    // m_squashingContainmentLayer
+    //   + m_graphicsLayer
+    //   + m_squashingLayer
+    OwnPtr<GraphicsLayer> m_squashingContainmentLayer; // Only used if any squashed layers exist and m_squashingContainmentLayer is not present, to contain the squashed layers as siblings to the rest of the GraphicsLayer tree chunk.
     OwnPtr<GraphicsLayer> m_squashingLayer; // Only used if any squashed layers exist, this is the backing that squashed layers paint into.
     Vector<GraphicsLayerPaintInfo> m_squashedLayers;
 
     LayoutRect m_compositedBounds;
     LayoutSize m_subpixelAccumulation; // The accumulated subpixel offset of the compositedBounds compared to absolute coordinates.
 
     bool m_artificiallyInflatedBounds : 1; // bounds had to be made non-zero to make transform-origin work
     bool m_isMainFrameRenderViewLayer : 1;
     bool m_requiresOwnBackingStoreForIntrinsicReasons : 1;
     bool m_requiresOwnBackingStoreForAncestorReasons : 1;
     bool m_canCompositeFilters : 1;
     bool m_backgroundLayerPaintsFixedRootBackground : 1;
     bool m_needToUpdateGeometry : 1;
     bool m_needToUpdateGeometryOfAllDecendants : 1;
 };
 
 } // namespace WebCore
 
 #endif // CompositedLayerMapping_h