Processes a successful controller into an HTTP 200 response.
Some controllers may not return a response object but simply the body of one. The VIEW event is called in that case, to allow us to mutate that body into a Response object. In particular we assume that the return from an HTML-type response is a render array from a legacy page callback and render it.
Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent $event: The Event to process.
public function onHtml(GetResponseForControllerResultEvent $event) {
$page_callback_result = $event
->getControllerResult();
return new Response(drupal_render_page($page_callback_result));
}