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

ash/wm/window_mirror_view.cc

 // 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.
 
 #include "ash/wm/window_mirror_view.h"
 
 #include "ash/aura/wm_window_aura.h"
 #include "ash/common/wm/forwarding_layer_delegate.h"
 #include "ash/common/wm/window_state.h"
 #include "ash/wm/window_state_aura.h"
@@ skipping 56 matching lines @@
 
 bool WindowMirrorView::GetNeedsNotificationWhenVisibleBoundsChange() const {
   return true;
 }
 
 void WindowMirrorView::OnVisibleBoundsChanged() {
   if (!layer_owner_ && !GetVisibleBounds().IsEmpty())
     InitLayerOwner();
 }
 
-ui::LayerDelegate* WindowMirrorView::CreateDelegate(
-    ui::LayerDelegate* delegate) {
-  if (!delegate)
+ui::LayerDelegate* WindowMirrorView::CreateDelegate(ui::Layer* new_layer,
+                                                    ui::Layer* old_layer) {
+  if (!old_layer || !old_layer->delegate())
     return nullptr;
   delegates_.push_back(
-      base::MakeUnique<ForwardingLayerDelegate>(target_, delegate));
-
+      base::MakeUnique<ForwardingLayerDelegate>(new_layer, old_layer));
   return delegates_.back().get();
 }
 
 void WindowMirrorView::InitLayerOwner() {
   if (!layer_owner_) {
     target_->aura_window()->SetProperty(aura::client::kMirroringEnabledKey,
                                         true);
   }
 
   layer_owner_ = ::wm::RecreateLayers(target_->aura_window(), this);
@@ skipping 20 matching lines @@
 gfx::Rect WindowMirrorView::GetClientAreaBounds() const {
   // The target window may not have a widget in unit tests.
   if (!target_->GetInternalWidget())
     return gfx::Rect();
   views::View* client_view = target_->GetInternalWidget()->client_view();
   return client_view->ConvertRectToWidget(client_view->GetLocalBounds());
 }
 
 }  // namespace wm
 }  // namespace ash