Support the serving of iframe content from a specific page

functions.php

//
// Wordpress - Support for iframes being called for a specific page. Add this to functions.php
// In this case, the URL $allowed_url will be allowed to pull the contents of the 'my-embed' page
//
add_action( 'send_headers', 'add_header_xframe', 99 );
function add_header_xframe() {
    if ( is_page('my-embed') ) {
        $allowed_url = 'http://test.local';
	    header( 'Content-Security-Policy: frame-ancestors '.$allowed_url );
    }
}