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

Source/core/rendering/compositing/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 221 matching lines @@
         // The absolute bounding rect of all the squashed layers.
         IntRect boundingRect;
 
         // This is simply the sum of the areas of the squashed rects. This can be very skewed if the rects overlap,
         // but should be close enough to drive a heuristic.
         uint64_t totalAreaOfSquashedRects;
     };
 
     bool hasUnresolvedDirtyBits();
 
+    enum NeedsSquashingBehavior {
+        ShouldPaintIntoOwnLayer = 0,
+        ShouldSquash = 1,
+        ShouldPaintIntoSameLayer = 2
+    };
+
     bool squashingWouldExceedSparsityTolerance(const RenderLayer* candidate, const SquashingState&);
-    bool canSquashIntoCurrentSquashingOwner(const RenderLayer* candidate, const SquashingState&);
+    NeedsSquashingBehavior computeSquashingBehaviorWhenSquashingRequired(const RenderLayer* candidate, const SquashingState&);
 
     CompositingStateTransitionType computeCompositedLayerUpdate(RenderLayer*);
     // Make updates to the layer based on viewport-constrained properties such as position:fixed. This can in turn affect
     // compositing.
     bool updateLayerIfViewportConstrained(RenderLayer*);
 
     // GraphicsLayerClient implementation
     virtual void notifyAnimationStarted(const GraphicsLayer*, double) OVERRIDE { }
     virtual void paintContents(const GraphicsLayer*, GraphicsContext&, GraphicsLayerPaintingPhase, const IntRect&) OVERRIDE;
 
@@ skipping 102 matching lines @@
     OwnPtr<GraphicsLayer> m_layerForVerticalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForScrollCorner;
 #if USE(RUBBER_BANDING)
     OwnPtr<GraphicsLayer> m_layerForOverhangShadow;
 #endif
 };
 
 } // namespace WebCore
 
 #endif // RenderLayerCompositor_h