mash: Migrate wallpaper controllers to ash/common.

ash/mus/test/wm_test_helper.h

 // Copyright 2016 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_MUS_TEST_WM_TEST_HELPER_H_
 #define ASH_MUS_TEST_WM_TEST_HELPER_H_
 
 #include <memory>
 
 #include "ash/mus/window_manager_application.h"
 #include "base/macros.h"
 #include "services/ui/public/cpp/tests/test_window_tree_client_setup.h"
 
 namespace base {
 class MessageLoop;
+class SequencedWorkerPoolOwner;
 }
 
 namespace gfx {
 class Rect;
 }
 
 namespace ui {
 class Window;
 }
 
@@ skipping 15 matching lines @@
     return window_manager_app_.get();
   }
 
   WmTestScreen* screen() { return screen_; }
 
  private:
   std::unique_ptr<base::MessageLoop> message_loop_;
   ui::TestWindowTreeClientSetup window_tree_client_setup_;
   std::unique_ptr<WindowManagerApplication> window_manager_app_;
   WmTestScreen* screen_ = nullptr;  // Owned by |window_manager_app_|.
+  std::unique_ptr<base::SequencedWorkerPoolOwner> blocking_pool_owner_;
 
   DISALLOW_COPY_AND_ASSIGN(WmTestHelper);
 };
 
 }  // namespace mus
 }  // namespace ash
 
 #endif  // ASH_MUS_TEST_WM_TEST_HELPER_H_