fix(UI): Fixed details on failure to load.

When switching from using document.createEvent to using the CustomEvent
constructor, we accidentally broke the "shaka-ui-load-failed" event
such that it no longer had a detail object. The detail object is meant
to be provided, not by itself, but as a member variable inside a custom
dictionary.
This fixes the syntax on the constructor.

Issue shaka-project#3388

Change-Id: I16dc025c21c8f0a012b5854bbbc56e8b110b2300

ui/ui.js

@@ -337,7 +337,7 @@ shaka.ui.Overlay = class {
         'reasonCode': reasonCode,
       };
     }
-    const uiLoadedEvent = new CustomEvent(eventName, detail);
+    const uiLoadedEvent = new CustomEvent(eventName, {detail});
     document.dispatchEvent(uiLoadedEvent);
   }