Update WM shadow transitions.

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 <memory>
 
 #include "base/macros.h"
@@ skipping 66 matching lines @@
   // The current style, set when the transition animation starts.
   Style style_ = STYLE_ACTIVE;
 
   // The parent layer of the shadow layer. It serves as a container accessible
   // from the outside to control the visibility of the shadow.
   std::unique_ptr<ui::Layer> layer_;
 
   // The actual shadow layer corresponding to a cc::NinePatchLayer.
   std::unique_ptr<ui::Layer> shadow_layer_;
 
+  // When the style changes, the old shadow cross-fades with the new one.
+  // When non-null, this is an old |shadow_layer_| that's being animated out.
+  std::unique_ptr<ui::Layer> fading_layer_;
+
   // Bounds of the content that the shadow encloses.
   gfx::Rect content_bounds_;
 
   DISALLOW_COPY_AND_ASSIGN(Shadow);
 };
 
 }  // namespace wm
 
 #endif  // UI_WM_CORE_SHADOW_H_