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

ui/wm/core/window_util.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_WINDOW_UTIL_H_
 #define UI_WM_CORE_WINDOW_UTIL_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 23 matching lines @@
 WM_EXPORT aura::Window* GetActivatableWindow(aura::Window* window);
 
 // Retrieves the toplevel window for |window|. The ActivationClient makes this
 // determination.
 WM_EXPORT aura::Window* GetToplevelWindow(aura::Window* window);
 
 // A factory method to create a delegate for recreated layers.
 class WM_EXPORT LayerDelegateFactory {
  public:
   virtual ~LayerDelegateFactory() = default;
-  // |original_delegate| may already be deleted by the time the new
+  // |original_layer| may already be deleted by the time the new
   // delegate is created, so if the new delegate has to access it
   // later, it is the new delegate's responsibility to make sure the
-  // original delegate is alive.
-  virtual ui::LayerDelegate* CreateDelegate(
-      ui::LayerDelegate* original_delegate) = 0;
+  // original layer/delegate is alive.
+  virtual ui::LayerDelegate* CreateDelegate(ui::Layer* new_layer,
+                                            ui::Layer* original_layer) = 0;
 };
 
 // Returns the existing Layer for |root| (and all its descendants) and creates
 // a new layer for |root| and all its descendants. This is intended for
 // animations that want to animate between the existing visuals and a new state.
 //
 // As a result of this |root| has freshly created layers, meaning the layers
 // have not yet been painted to.
 //
 // When a non null |delegate_factory| is passed, it will be used to
@@ skipping 14 matching lines @@
 WM_EXPORT void RemoveTransientChild(aura::Window* parent, aura::Window* child);
 
 // Returns true if |window| has |ancestor| as a transient ancestor. A transient
 // ancestor is found by following the transient parent chain of the window.
 WM_EXPORT bool HasTransientAncestor(const aura::Window* window,
                                     const aura::Window* ancestor);
 
 }  // namespace wm
 
 #endif  // UI_WM_CORE_WINDOW_UTIL_H_