mash: Migrate wallpaper controllers to ash/common.

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index eae388f9178c11178ed1afe5e5d1920c39d651cd..28d6e335b28b75d2b6d1bb47be3192ad0fcd4835 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -128,7 +128,6 @@ class ScreenLayoutObserver;
 class VirtualKeyboardController;
 class VideoActivityNotifier;
 class VideoDetector;
-class WallpaperController;
 class WebNotificationTray;
 class WindowPositioner;
 class WindowTreeHostManager;
@@ -274,9 +273,6 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
     return tooltip_controller_.get();
   }
   OverlayEventFilter* overlay_filter() { return overlay_filter_.get(); }
-  WallpaperController* wallpaper_controller() {
-    return wallpaper_controller_.get();
-  }
   LinkHandlerModelFactory* link_handler_model_factory() {
     return link_handler_model_factory_;
   }
@@ -500,7 +496,6 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   std::unique_ptr<::wm::VisibilityController> visibility_controller_;
   std::unique_ptr<::wm::WindowModalityController> window_modality_controller_;
   std::unique_ptr<views::corewm::TooltipController> tooltip_controller_;
-  std::unique_ptr<WallpaperController> wallpaper_controller_;
   LinkHandlerModelFactory* link_handler_model_factory_;
   std::unique_ptr<PowerButtonController> power_button_controller_;
   std::unique_ptr<LockStateController> lock_state_controller_;