Moves ShellObserver to ash/common

ash/desktop_background/desktop_background_controller.h

 // 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.
 
 #ifndef ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_CONTROLLER_H_
 #define ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_CONTROLLER_H_
 
 #include <memory>
 
 #include "ash/ash_export.h"
+#include "ash/common/shell_observer.h"
 #include "ash/display/window_tree_host_manager.h"
-#include "ash/shell_observer.h"
 #include "base/files/file_path.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
 #include "base/timer/timer.h"
 #include "components/wallpaper/wallpaper_layout.h"
 #include "ui/compositor/layer.h"
 #include "ui/gfx/image/image_skia.h"
 
@@ skipping 143 matching lines @@
   int wallpaper_reload_delay_;
 
   base::SequencedWorkerPool* blocking_pool_;
 
   DISALLOW_COPY_AND_ASSIGN(DesktopBackgroundController);
 };
 
 }  // namespace ash
 
 #endif  // ASH_DESKTOP_BACKGROUND_DESKTOP_BACKGROUND_CONTROLLER_H_