authPrompt: Keep buttonWell and cancel button sizes in sync

So that the entry is horizontally centralized at all times.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/922

js/gdm/authPrompt.js

@@ -176,6 +176,10 @@ var AuthPrompt = GObject.registerClass({
             x_align: Clutter.ActorAlign.END,
             y_align: Clutter.ActorAlign.CENTER,
         });
+        this._defaultButtonWell.add_constraint(new Clutter.BindConstraint({
+            source: this.cancelButton,
+            coordinate: Clutter.BindCoordinate.SIZE,
+        }));
         this._mainBox.add_child(this._defaultButtonWell);
 
         this._spinner = new Animation.Spinner(DEFAULT_BUTTON_WELL_ICON_SIZE);