Allow layers that need squashing to draw into the squashing layer in some cases.

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index 1ab02f0ea745cd94ec2b3d9722b3236b3543d09d..153940bc1c793ca25b3795cb15647231446c1ee7 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -247,6 +247,7 @@ public:
     RenderLayer* enclosingCompositingLayerForRepaint(IncludeSelfOrNot = IncludeSelf) const;
     // Ancestor compositing layer, excluding this.
     RenderLayer* ancestorCompositingLayer() const { return enclosingCompositingLayer(ExcludeSelf); }
+    RenderLayer* containingGroupedLayer() const;
 
     // Ancestor composited scrolling layer at or above our containing block.
     RenderLayer* ancestorCompositedScrollingLayer() const;