dateMenu: Sync initial state of the message indicator

While the unread messages indicator is updated when starting a new
session because we call _onSourceAdded() on existing sources, we should
also update the do-not-disturb setting which might still be enabled.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1037

js/ui/dateMenu.js

@@ -454,6 +454,8 @@ class MessagesIndicator extends St.Icon {
         let sources = Main.messageTray.getSources();
         sources.forEach(source => this._onSourceAdded(null, source));
 
+        this._sync();
+
         this.connect('destroy', () => {
             this._settings.run_dispose();
             this._settings = null;