Updates a part of the add view form via AJAX.
The part of the form that has changed.
function views_ui_ajax_update_form($form, $form_state) {
// The region that needs to be updated was stored in a property of the
// triggering element by views_ui_add_ajax_trigger(), so all we have to do is
// retrieve that here.
return drupal_array_get_nested_value($form, $form_state['triggering_element']['#views_ui_ajax_data']['refresh_parents']);
}