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 3e0960d24b87e2e090dc53dc8c1ee6d0e9e8047c..f4ddcacdf8a8919e604a4a2273e5dba6d8627741 100644
--- a/ash/wm/system_modal_container_layout_manager.cc
+++ b/ash/wm/system_modal_container_layout_manager.cc
@@ -177,8 +177,8 @@ void SystemModalContainerLayoutManager::DestroyModalBackground() {
     ui::ScopedLayerAnimationSettings settings(
         modal_background_->GetNativeView()->layer()->GetAnimator());
     modal_background_->Close();
-    settings.AddObserver(views::corewm::CreateHidingWindowAnimationObserver(
-        modal_background_->GetNativeView()));
+    views::corewm::DetachLayersForHidingAnimationWhenDestroyed(
+        modal_background_->GetNativeView(), &settings);
     modal_background_->GetNativeView()->layer()->SetOpacity(0.0f);
     modal_background_ = NULL;
   }