Set up clip and scroll parents on the blink side.

Source/core/rendering/RenderLayerCompositor.h

 /*
  * Copyright (C) 2009 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 86 matching lines @@
 
     // Update the compositing state of the given layer. Returns true if that state changed.
     enum CompositingChangeRepaint { CompositingChangeRepaintNow, CompositingChangeWillRepaintLater };
     bool updateLayerCompositingState(RenderLayer*, CompositingChangeRepaint = CompositingChangeRepaintNow);
 
     // Update the geometry for compositing children of compositingAncestor.
     void updateCompositingDescendantGeometry(RenderLayer* compositingAncestor, RenderLayer*, bool compositedChildrenOnly);
 
     // Whether layer's backing needs a graphics layer to do clipping by an ancestor (non-stacking-context parent with overflow).
     bool clippedByAncestor(RenderLayer*) const;
+    // Whether a layer's backing needs an extra graphics layer to do clipping by a scroll parent.
+    bool clippedByScrollingAncestor(RenderLayer*) const;

[enne (OOO), 2013/08/22 20:54:30]

I see.  This is your solution to the ordering issue that you talked about
previously.  By adding an additional clipping layer that's an ancestor to all of
the scroll children you're guaranteed that it comes before them in the tree. 
Neat!

     // Whether layer's backing needs a graphics layer to clip z-order children of the given layer.
     bool clipsCompositingDescendants(const RenderLayer*) const;
 
     // Whether the given layer needs an extra 'contents' layer.
     bool needsContentsCompositingLayer(const RenderLayer*) const;
 
     bool supportsFixedRootBackgroundCompositing() const;
     bool needsFixedRootBackgroundLayer(const RenderLayer*) const;
     GraphicsLayer* fixedRootBackgroundLayer() const;
 
@@ skipping 243 matching lines @@
     int m_secondaryCompositedLayerCount; // count of layers that have to be composited because of stacking or overlap.
     double m_obligatoryBackingStoreBytes;
     double m_secondaryBackingStoreBytes;
 #endif
 };
 
 
 } // namespace WebCore
 
 #endif // RenderLayerCompositor_h