Hide shadow layer when border exceeds layer bounds.

ui/wm/core/shadow.h

Index: ui/wm/core/shadow.h
diff --git a/ui/wm/core/shadow.h b/ui/wm/core/shadow.h
index 6734445a05f38c223e9aa64d544c29309045e410..8468c67a092a218995852b27b56561b51d6cd14b 100644
--- a/ui/wm/core/shadow.h
+++ b/ui/wm/core/shadow.h
@@ -67,8 +67,13 @@ class WM_EXPORT Shadow : public ui::ImplicitAnimationObserver {
   // 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_;