messageTray: Don't create notification policy on demand

This was changed in commit 8f15193b4 as a work-around for an ES6
class limitation, but now that Sources are GObject subclasses, we
no longer need to use that somewhat surprising pattern.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1131

js/ui/windowAttentionHandler.js

@@ -57,11 +57,11 @@ var WindowAttentionHandler = class {
 var WindowAttentionSource = GObject.registerClass(
 class WindowAttentionSource extends MessageTray.Source {
     _init(app, window) {
-        super._init(app.get_name());
-
         this._window = window;
         this._app = app;
 
+        super._init(app.get_name());
+
         this.signalIDs = [];
         this.signalIDs.push(this._window.connect('notify::demands-attention',
                                                  this._sync.bind(this)));