Reveal hidden submenus on page load – Twenty Fifteen theme

Figured it out! Use the following PHP code.

/*
 * Toggles the menu dropdowns to open state
*/
function mod_toggle_menus() {
    ?>
    <script type="text/javascript">
    jQuery(document).ready( function() {

        /* add toggled-on class to submenus */
        jQuery('.sub-menu').addClass('toggled-on');

    });
    </script>
    <?php
}
add_action('wp_footer' , 'mod_toggle_menus');