Hide shadow layer when border exceeds layer bounds.

ui/wm/core/shadow.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_WM_CORE_SHADOW_H_
 #define UI_WM_CORE_SHADOW_H_
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "ui/compositor/layer_animation_observer.h"
@@ skipping 49 matching lines @@
   // Updates the shadow images to the current |style_|.
   void UpdateImagesForStyle();
 
   // Updates the shadow layer bounds based on the inteior inset and the current
   // |content_bounds_|.
   void UpdateLayerBounds();
 
   // The current style, set when the transition animation starts.
   Style style_;
 
+  // The parent layer of the shadow layer. It serves as a container accessible
+  // from the outside to control the visibility of the shadow.
   scoped_ptr<ui::Layer> layer_;
 
+  // The actual shadow layer corresponding to a cc::NinePatchLayer.
+  scoped_ptr<ui::Layer> shadow_layer_;
+
   // Bounds of the content that the shadow encloses.
   gfx::Rect content_bounds_;
 
   // The interior inset of the shadow images. The content bounds of the image
   // grid should be set to |content_bounds_| inset by this amount.
   int interior_inset_;
 
   DISALLOW_COPY_AND_ASSIGN(Shadow);
 };
 
 }  // namespace wm
 
 #endif  // UI_WM_CORE_SHADOW_H_