Adding Depth to WordPress Sub Menu

header.php

<?php
/**
 * In header.php
 */

      <nav id="site-navigation" class="main-navigation">
        <button class="menu-toggle" aria-controls="primary-menu" aria-expanded="false"><?php esc_html_e('Primary Menu', 'custom_walker'); ?></button>
        <?php
        wp_nav_menu(
          array(
            'theme_location' => 'menu-1',
            'menu_id'        => 'primary-menu',
            'walker'          => new Primary_Walker_Nav_Menu()
          )
        );
        ?>
      </nav><!-- #site-navigation -->

wordpress-walker-sub-menu.php

<?php
/**
 * In functions.php
 */

class Primary_Walker_Nav_Menu extends Walker_Nav_Menu
{
  function start_lvl(&$output, $depth = 0, $args = array())
  {
    $indent = str_repeat("\t", $depth);
    $output .= "\n$indent<ul class=\"sub-menu level-$depth\" role=\"menu\">\n";
  }
}