Keep dedicated layers for hiding animation

ash/wm/system_modal_container_layout_manager.cc

Index: ash/wm/system_modal_container_layout_manager.cc
diff --git a/ash/wm/system_modal_container_layout_manager.cc b/ash/wm/system_modal_container_layout_manager.cc
index a1432c6b9cfb3acfe3075d068d70895a725f87fa..69375010c7fd3477e5e50ef7e92441743d440d0c 100644
--- a/ash/wm/system_modal_container_layout_manager.cc
+++ b/ash/wm/system_modal_container_layout_manager.cc
@@ -176,11 +176,9 @@ 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());
+    ::wm::ScopedHidingAnimationSettings settings(
+        modal_background_->GetNativeView());
     modal_background_->Close();
-    settings.AddObserver(::wm::CreateHidingWindowAnimationObserver(
-        modal_background_->GetNativeView()));
     modal_background_->GetNativeView()->layer()->SetOpacity(0.0f);
     modal_background_ = NULL;
   }