Update WM shadow transitions.

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 e0b7a1db301c88aa440c6d48f9091a6ef4348a72..24fbfed94d6308f935c0c508dd11863b6d4af879 100644
--- a/ui/wm/core/shadow.h
+++ b/ui/wm/core/shadow.h
@@ -62,8 +62,8 @@ class WM_EXPORT Shadow : public ui::ImplicitAnimationObserver {
   void OnImplicitAnimationsCompleted() override;
 
  private:
-  // Updates the shadow images to the current |style_|.
-  void UpdateImagesForStyle();
+  // Updates the shadow layer and its image to the current |style_|.
+  void RecreateShadowLayer();
 
   // Updates the shadow layer bounds based on the inteior inset and the current
   // |content_bounds_|.
@@ -84,6 +84,10 @@ class WM_EXPORT Shadow : public ui::ImplicitAnimationObserver {
   // 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_;