Revert of Split Screen mode implementation

athena/wm/public/window_manager_observer.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ATHENA_SCREEN_PUBLIC_WINDOW_MANAGER_OBSERVER_H_
 #define ATHENA_SCREEN_PUBLIC_WINDOW_MANAGER_OBSERVER_H_
 
 #include "athena/athena_export.h"
 
 namespace athena {
 
 class ATHENA_EXPORT WindowManagerObserver {
  public:
   virtual ~WindowManagerObserver() {}
 
-  // Called immediately before the overview mode is displayed.
+  // Called when the overview mode is displayed.
   virtual void OnOverviewModeEnter() = 0;
 
-  // Called immediately after going out of the overview mode.
+  // Called when going out of overview mode.
   virtual void OnOverviewModeExit() = 0;
 };
 
 }  // namespace athena
 
 #endif  // ATHENA_SCREEN_PUBLIC_WINDOW_MANAGER_OBSERVER_H_