ash: Add missing include to test from r236852.

ash/desktop_background/desktop_background_controller_unittest.cc

 // Copyright (c) 2012 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.
 
 #include "ash/desktop_background/desktop_background_controller.h"
 
 #include <cmath>
 #include <cstdlib>
 
 #include "ash/ash_switches.h"
 #include "ash/desktop_background/desktop_background_controller_observer.h"
 #include "ash/desktop_background/desktop_background_widget_controller.h"
 #include "ash/root_window_controller.h"
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "ash/test/ash_test_base.h"
 #include "ash/test/display_manager_test_api.h"
 #include "ash/test/test_user_wallpaper_delegate.h"
 #include "base/command_line.h"
 #include "base/file_util.h"
 #include "base/files/file_path.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/message_loop/message_loop.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/test/test_browser_thread.h"
 #include "content/public/test/test_utils.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/aura/root_window.h"
 #include "ui/compositor/scoped_animation_duration_scale_mode.h"
 #include "ui/compositor/test/layer_animator_test_controller.h"
 #include "ui/gfx/codec/jpeg_codec.h"
 #include "ui/gfx/point.h"
@@ skipping 624 matching lines @@
       DesktopBackgroundController::GetMaxDisplaySizeInNative().ToString());
 
   // Maximum width and height belongs to different displays.
   UpdateDisplay("400x300,100x500");
   EXPECT_EQ(
       "400x500",
       DesktopBackgroundController::GetMaxDisplaySizeInNative().ToString());
 }
 
 }  // namespace ash