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 42cbb05dd8ede6e2fde35c7d4bd9a7123e2d956f..53a75ea9b4a6331d818525b96cf751bf974bba57 100644
--- a/ash/wm/system_modal_container_layout_manager.cc
+++ b/ash/wm/system_modal_container_layout_manager.cc
@@ -174,11 +174,9 @@ 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());
+    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;
   }