Add ui::LayerObserver and use it to update Alt+Tab previews as needed.

ash/wm/window_mirror_view.h

 // Copyright 2016 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 ASH_WM_WINDOW_MIRROR_VIEW_H_
 #define ASH_WM_WINDOW_MIRROR_VIEW_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 18 matching lines @@
   explicit WindowMirrorView(WmWindowAura* window);
   ~WindowMirrorView() override;
 
   // views::View:
   gfx::Size GetPreferredSize() const override;
   void Layout() override;
   bool GetNeedsNotificationWhenVisibleBoundsChange() const override;
   void OnVisibleBoundsChanged() override;
 
   // ::wm::LayerDelegateFactory:
-  ui::LayerDelegate* CreateDelegate(ui::LayerDelegate* delegate) override;
+  ui::LayerDelegate* CreateDelegate(ui::Layer* new_layer,
+                                    ui::Layer* layer) override;
 
  private:
   void InitLayerOwner();
 
   // Gets the root of the layer tree that was lifted from |target_| (and is now
   // a child of |this->layer()|).
   ui::Layer* GetMirrorLayer();
 
   // Calculates the bounds of the client area of the Window in the widget
   // coordinate space.
   gfx::Rect GetClientAreaBounds() const;
 
   // The original window that is being represented by |this|.
   WmWindowAura* target_;
 
   // Retains ownership of the mirror layer tree. This is lazily initialized
   // the first time the view becomes visible.
   std::unique_ptr<ui::LayerTreeOwner> layer_owner_;
 
   std::vector<std::unique_ptr<ForwardingLayerDelegate>> delegates_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowMirrorView);
 };
 
 }  // namespace wm
 }  // namespace ash
 
 #endif  // ASH_WM_WINDOW_MIRROR_VIEW_H_