animation: fix unintentional loop while polkit dialog is active

The polkit password dialog has a spinner that gets displayed
while the users password is being verified.

Unfortunately, the spinner stop method unintentionally calls
back into itself after the stop fade out animation is complete.
The stop method is called at startup, so the looping begins as
soon as the dialog is visible and continues until the dialog is
dismissed.

This commit fixes the loop by having the stop method cease
calling itself, and instead having it call the stop method on the
superclass.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/602

js/ui/animation.js

@@ -162,7 +162,7 @@ var Spinner = class extends AnimatedIcon {
                 time: SPINNER_ANIMATION_TIME,
                 transition: 'linear',
                 onComplete: () => {
-                    this.stop(false);
+                    super.stop();
                 }
             });
         } else {