Set up clip and scroll parents on the blink side.

Source/core/rendering/RenderLayerCompositor.h

Index: Source/core/rendering/RenderLayerCompositor.h
diff --git a/Source/core/rendering/RenderLayerCompositor.h b/Source/core/rendering/RenderLayerCompositor.h
index 0d12aa49a8f4560a6d383ce6a426ce0479e6d570..248003a4f3a70e34c0156489a03960d06adf04af 100644
--- a/Source/core/rendering/RenderLayerCompositor.h
+++ b/Source/core/rendering/RenderLayerCompositor.h
@@ -104,6 +104,8 @@ public:
 
     // 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;