Use OnceCallback on Mojo interfaces in //ash

ash/shell/window_watcher_shelf_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_SHELL_WINDOW_WATCHER_SHELF_ITEM_DELEGATE_H_
 #define ASH_SHELL_WINDOW_WATCHER_SHELF_ITEM_DELEGATE_H_
 
 #include "ash/public/cpp/shelf_item.h"
 #include "ash/public/cpp/shelf_item_delegate.h"
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 
 namespace ash {
 namespace shell {
 
 class WindowWatcher;
 
 // ShelfItemDelegate implementation used by WindowWatcher.
 class WindowWatcherShelfItemDelegate : public ShelfItemDelegate {
  public:
   WindowWatcherShelfItemDelegate(ShelfID id, WindowWatcher* watcher);
   ~WindowWatcherShelfItemDelegate() override;
 
   // ShelfItemDelegate:
   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;
 
  private:
   WindowWatcher* watcher_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowWatcherShelfItemDelegate);
 };
 
 }  // namespace shell
 }  // namespace ash
 
 #endif  // ASH_SHELL_WINDOW_WATCHER_SHELF_ITEM_DELEGATE_H_