Add Blink-side flag for enabling/disabling layer squashing.

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 57 matching lines @@
     // composited RenderLayers)
     bool inCompositingMode() const { return m_compositing; }
     // This will make a compositing layer at the root automatically, and hook up to
     // the native view/window system.
     void enableCompositingMode(bool enable = true);
 
     bool inForcedCompositingMode() const { return m_forceCompositingMode; }
 
     // Returns true if the accelerated compositing is enabled
     bool hasAcceleratedCompositing() const { return m_hasAcceleratedCompositing; }
+    bool isLayerSquashingEnabled() const;
 
     bool canRender3DTransforms() const;
 
     // Copy the accelerated compositing related flags from Settings
     void cacheAcceleratedCompositingFlags();
 
     // Called when the layer hierarchy needs to be updated (compositing layers have been
     // created, destroyed or re-parented).
     void setCompositingLayersNeedRebuild(bool needRebuild = true);
     bool compositingLayersNeedRebuild() const { return m_compositingLayersNeedRebuild; }
@@ skipping 255 matching lines @@
     OwnPtr<GraphicsLayer> m_layerForScrollCorner;
 #if USE(RUBBER_BANDING)
     OwnPtr<GraphicsLayer> m_layerForOverhangShadow;
 #endif
 };
 
 
 } // namespace WebCore
 
 #endif // RenderLayerCompositor_h