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>
 
 #include "ash/ash_export.h"
 #include "base/macros.h"
+#include "base/scoped_observer.h"
+#include "ui/compositor/layer_observer.h"
 #include "ui/views/view.h"
 #include "ui/wm/core/window_util.h"
 
 namespace ash {
 
 class WmWindowAura;
 
 namespace wm {
 
 class ForwardingLayerDelegate;
 
 // A view that mirrors the client area of a single window. Layers are lifted
 // from the underlying window (which gets new ones in their place). New paint
 // calls, if any, are forwarded to the underlying window.
-class WindowMirrorView : public views::View, public ::wm::LayerDelegateFactory {
+class WindowMirrorView : public views::View,
+                         public ::wm::LayerDelegateFactory,
+                         public ui::LayerObserver {
  public:
   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::LayerObserver:
+  void OnDelegatedFrameDamageInTree() override;
+  void OnLayerDestroyed(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_;
 
+  ScopedObserver<ui::Layer, ui::LayerObserver> scoped_observer_;
+
   std::vector<std::unique_ptr<ForwardingLayerDelegate>> delegates_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowMirrorView);
 };
 
 }  // namespace wm
 }  // namespace ash
 
 #endif  // ASH_WM_WINDOW_MIRROR_VIEW_H_