Keep dedicated layers for hiding animation

ash/wm/window_animations.cc

Index: ash/wm/window_animations.cc
diff --git a/ash/wm/window_animations.cc b/ash/wm/window_animations.cc
index f5328f94501e35b0041f30fd58fbe025f34f634b..2a92c1107a1eb07e29091650a6adf31ff2895251 100644
--- a/ash/wm/window_animations.cc
+++ b/ash/wm/window_animations.cc
@@ -176,8 +176,9 @@ void AnimateHideWindow_Minimize(aura::Window* window) {
   base::TimeDelta duration = base::TimeDelta::FromMilliseconds(
       kLayerAnimationsForMinimizeDurationMS);
   settings.SetTransitionDuration(duration);
-  settings.AddObserver(
-      views::corewm::CreateHidingWindowAnimationObserver(window));
+  scoped_ptr<views::corewm::LayerDetacherForHidingAnimation> detacher =
+      views::corewm::DetachAndRecreateLayersForHidingAnimation(
+          window, &settings).Pass();
   window->layer()->SetVisible(false);
 
   AddLayerAnimationsForMinimize(window, false);
@@ -209,8 +210,8 @@ void AnimateShowHideWindowCommon_BrightnessGrayscale(aura::Window* window,
   ui::ScopedLayerAnimationSettings settings(window->layer()->GetAnimator());
   settings.SetTransitionDuration(duration);
   if (!show) {
-    settings.AddObserver(
-        views::corewm::CreateHidingWindowAnimationObserver(window));
+    views::corewm::DetachLayersForHidingAnimationWhenDestroyed(
+        window, &settings);
   }
 
   window->layer()->GetAnimator()->