Reland of [ash-md] Fades overview header in and out

ash/common/wm/overview/scoped_overview_animation_settings.h

Index: ash/common/wm/overview/scoped_overview_animation_settings.h
diff --git a/ash/common/wm/overview/scoped_overview_animation_settings.h b/ash/common/wm/overview/scoped_overview_animation_settings.h
index 78d184d4f1295b81bdd5455866cb0e92b345ea8e..a5176ede7f7ece2ca4f0eacf03385752f7f374ab 100644
--- a/ash/common/wm/overview/scoped_overview_animation_settings.h
+++ b/ash/common/wm/overview/scoped_overview_animation_settings.h
@@ -5,6 +5,10 @@
 #ifndef ASH_COMMON_WM_OVERVIEW_SCOPED_OVERVIEW_ANIMATION_SETTINGS_H_
 #define ASH_COMMON_WM_OVERVIEW_SCOPED_OVERVIEW_ANIMATION_SETTINGS_H_
 
+namespace ui {
+class ImplicitAnimationObserver;
+}  // namespace ui
+
 namespace ash {
 
 // ScopedOverviewAnimationSettings correctly configures the animation
@@ -12,6 +16,7 @@ namespace ash {
 class ScopedOverviewAnimationSettings {
  public:
   virtual ~ScopedOverviewAnimationSettings() {}
+  virtual void AddObserver(ui::ImplicitAnimationObserver* observer) = 0;
 };
 
 }  // namespace ash