Assign `extras` from the top-level of glTF definition to `userData` of the result object.

examples/js/loaders/GLTFLoader.js

@@ -1578,6 +1578,8 @@ THREE.GLTFLoader = ( function () {
 
 			addUnknownExtensionsToUserData( extensions, result, json );
 
+			assignExtrasToUserData( result, json );
+
 			onLoad( result );
 
 		} ).catch( onError );