aura: Implement new app list mock.

ash/app_list/app_list_view_delegate.h

 // Copyright (c) 2011 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_APP_LIST_APP_LIST_VIEW_DELEGATE_H_
 #define ASH_APP_LIST_APP_LIST_VIEW_DELEGATE_H_
 #pragma once
 
+#include <string>
+
 #include "ash/ash_export.h"
 
 namespace ash {
 
 class AppListItemModel;
+class AppListModel;
 
 class ASH_EXPORT AppListViewDelegate {
  public:
   // AppListView owns the delegate.
   virtual ~AppListViewDelegate() {}
 
+  // Invoked to ask the delegate to populate the |model| for given |query|.
+  virtual void BuildAppListModel(const std::string& query,
+                                 AppListModel* model) = 0;
+
   // Invoked an AppListeItemModelView is  activated by click or enter key.
   virtual void OnAppListItemActivated(AppListItemModel* item,
                                       int event_flags) = 0;
 };
 
 }  // namespace ash
 
 #endif  // ASH_APP_LIST_APP_LIST_VIEW_DELEGATE_H_