Keep dedicated layers for hiding animation

ash/wm/system_modal_container_layout_manager.cc

 // 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.
 
 #include "ash/wm/system_modal_container_layout_manager.h"
 
 #include "ash/session_state_delegate.h"
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "ash/wm/system_modal_container_event_filter.h"
@@ skipping 156 matching lines @@
       modal_background_->GetNativeView()->layer()->GetAnimator());
   modal_background_->Show();
   modal_background_->GetNativeView()->layer()->SetOpacity(0.5f);
   container_->StackChildAtTop(modal_background_->GetNativeView());
 }
 
 void SystemModalContainerLayoutManager::DestroyModalBackground() {
   // modal_background_ can be NULL when a root window is shutting down
   // and OnWindowDestroying is called first.
   if (modal_background_) {
-    ui::ScopedLayerAnimationSettings settings(
-        modal_background_->GetNativeView()->layer()->GetAnimator());
+    views::corewm::ScopedHidingAnimationSettings settings(
+        modal_background_->GetNativeView());
     modal_background_->Close();
-    settings.AddObserver(views::corewm::CreateHidingWindowAnimationObserver(
-        modal_background_->GetNativeView()));
     modal_background_->GetNativeView()->layer()->SetOpacity(0.0f);
     modal_background_ = NULL;
   }
 }
 
 // static
 bool SystemModalContainerLayoutManager::IsModalBackground(
     aura::Window* window) {
   int id = window->parent()->id();
   if (id != internal::kShellWindowId_SystemModalContainer &&
@@ skipping 23 matching lines @@
 void SystemModalContainerLayoutManager::RemoveModalWindow(
     aura::Window* window) {
   aura::Window::Windows::iterator it =
       std::find(modal_windows_.begin(), modal_windows_.end(), window);
   if (it != modal_windows_.end())
     modal_windows_.erase(it);
 }
 
 }  // namespace internal
 }  // namespace ash