Revert 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 a5176ede7f7ece2ca4f0eacf03385752f7f374ab..78d184d4f1295b81bdd5455866cb0e92b345ea8e 100644
--- a/ash/common/wm/overview/scoped_overview_animation_settings.h
+++ b/ash/common/wm/overview/scoped_overview_animation_settings.h
@@ -4,10 +4,6 @@
 
 #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 {
 
@@ -16,7 +12,6 @@
 class ScopedOverviewAnimationSettings {
  public:
   virtual ~ScopedOverviewAnimationSettings() {}
-  virtual void AddObserver(ui::ImplicitAnimationObserver* observer) = 0;
 };
 
 }  // namespace ash