Fix UI TextDisplayer instantiation

You can't use "new" with an arrow function, and the
TextDisplayerFactory is currently called with "new" (until #1521 is
resolved, and we stop using "new" with factories).

Change-Id: I365ff46dc59dcc0f9a751918567557b99366c950

ui/ui.js

@@ -241,7 +241,7 @@ shaka.ui.Overlay.createUI_ = function(container, video, config) {
   // If the browser's native controls are disabled, use UI TextDisplayer.
   if (!video.controls) {
     player.configure('textDisplayFactory',
-      () => new shaka.ui.TextDisplayer(video, container));
+        function() { return new shaka.ui.TextDisplayer(video, container); });
   }
 
   container['ui'] = ui;