background: Pass cancellable when querying file info

Otherwise it is possible that the operation completes successfully
after the background has been destroyed, which will throw a warning
(caused by accessing the this._fileWatches object after it has been
nulled).

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5337

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2268>

js/ui/background.js

@@ -492,7 +492,7 @@ var Background = GObject.registerClass({
                 Gio.FILE_ATTRIBUTE_STANDARD_CONTENT_TYPE,
                 Gio.FileQueryInfoFlags.NONE,
                 0,
-                null);
+                this._cancellable);
         } catch (e) {
             this._setLoaded();
             return;