screenShield: Chain up Arrow's style_changed vfunc

This makes style changes propagate to our child widget as they
should.

https://bugzilla.gnome.org/show_bug.cgi?id=739178

js/ui/screenShield.js

@@ -399,6 +399,8 @@ const Arrow = new Lang.Class({
             this._shadowHelper = St.ShadowHelper.new(this._shadow);
         else
             this._shadowHelper = null;
+
+        this.parent();
     },
 
     vfunc_paint: function() {