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 51 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::WrapUnique(new ForwardingLayerDelegate(target_, delegate)));
-
+      base::WrapUnique(new ForwardingLayerDelegate(new_layer, old_layer)));
   return delegates_.back().get();
 }
 
 void WindowMirrorView::InitLayerOwner() {
   SetPaintToLayer(true);
 
   layer_owner_ = ::wm::RecreateLayers(target_->aura_window(), this);
 
   GetMirrorLayer()->parent()->Remove(GetMirrorLayer());
   layer()->Add(GetMirrorLayer());
   // This causes us to clip the non-client areas of the window.
   layer()->SetMasksToBounds(true);
 
   // Some extra work is needed when the target window is minimized.
   if (target_->GetWindowState()->IsMinimized()) {
-    GetMirrorLayer()->SetVisible(true);
     GetMirrorLayer()->SetOpacity(1);
     EnsureAllChildrenAreVisible(GetMirrorLayer());
   }
 
   Layout();
 }
 
 ui::Layer* WindowMirrorView::GetMirrorLayer() {
   return layer_owner_->root();
 }
 
 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