WooCommerce - Allow rendering of checkout and account pages in iframes

functions.php

<?php // Do not include this if already open!

/**
 * Code goes in theme functions.php.
 */
add_action( 'after_setup_theme', 'wc_remove_frame_options_header', 11 );

/**
 * Allow rendering of checkout and account pages in iframes.
 */
function wc_remove_frame_options_header() {
	remove_action( 'template_redirect', 'wc_send_frame_options_header' );
}