Revert of Split Screen mode implementation

athena/wm/public/window_manager.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_H_
 #define ATHENA_SCREEN_PUBLIC_WINDOW_MANAGER_H_
 
 #include "athena/athena_export.h"
 
 namespace athena {
 
 class WindowManagerObserver;
 
 // Manages the application, web windows.
 class ATHENA_EXPORT WindowManager {
  public:
   // Creates and deletes the singleton object of the WindowManager
   // implementation.
   static WindowManager* Create();
   static void Shutdown();
   static WindowManager* GetInstance();
 
   virtual ~WindowManager() {}
 
   virtual void ToggleOverview() = 0;
 
-  virtual bool IsOverviewModeActive() = 0;
-
   virtual void AddObserver(WindowManagerObserver* observer) = 0;
   virtual void RemoveObserver(WindowManagerObserver* observer) = 0;
 };
 
 }  // namespace athena
 
 #endif  // ATHENA_SCREEN_PUBLIC_WINDOW_MANAGER_H_