mash: Move ShelfWindowWatcher[ItemDelegate] to ash/common.

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index df71a2985e590cdee7cf1ebda3866ba0d81ed24e..542d572504ce771ed2dc091d20643c81e6666360 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -117,7 +117,6 @@ class ScreenPinningController;
 class ScreenPositionController;
 class SessionStateDelegate;
 class Shelf;
-class ShelfWindowWatcher;
 class ShellDelegate;
 struct ShellInitParams;
 class SlowAnimationEventFilter;
@@ -523,7 +522,6 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   std::unique_ptr<UserWallpaperDelegate> user_wallpaper_delegate_;
   std::unique_ptr<SessionStateDelegate> session_state_delegate_;
   std::unique_ptr<PointerWatcherDelegate> pointer_watcher_delegate_;
-  std::unique_ptr<ShelfWindowWatcher> shelf_window_watcher_;
   std::unique_ptr<WindowPositioner> window_positioner_;
 
   std::unique_ptr<DragDropController> drag_drop_controller_;