Use OnceCallback on Mojo interfaces in //ash

ash/shelf/shelf_window_watcher_item_delegate.h

 // Copyright 2013 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 ASH_SHELF_SHELF_WINDOW_WATCHER_ITEM_DELEGATE_H_
 #define ASH_SHELF_SHELF_WINDOW_WATCHER_ITEM_DELEGATE_H_
 
 #include "ash/public/cpp/shelf_item_delegate.h"
 #include "base/macros.h"
 
 namespace ash {
 
 class WmWindow;
 
 // ShelfItemDelegate for the items created by ShelfWindowWatcher, for example:
 // The Chrome OS settings window, task manager window, and panel windows.
 class ShelfWindowWatcherItemDelegate : public ShelfItemDelegate {
  public:
   ShelfWindowWatcherItemDelegate(const ShelfID& id, WmWindow* window);
   ~ShelfWindowWatcherItemDelegate() override;
 
  private:
   // ShelfItemDelegate overrides:
   void ItemSelected(std::unique_ptr<ui::Event> event,
                     int64_t display_id,
                     ShelfLaunchSource source,
-                    const ItemSelectedCallback& callback) override;
+                    ItemSelectedCallback callback) override;
   void ExecuteCommand(uint32_t command_id, int32_t event_flags) override;
   void Close() override;
 
   // The window associated with this item. Not owned.
   WmWindow* window_;
 
   DISALLOW_COPY_AND_ASSIGN(ShelfWindowWatcherItemDelegate);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SHELF_SHELF_WINDOW_WATCHER_ITEM_DELEGATE_H_