chromium/third_party/blink/web_tests/http/tests/misc/resources/slowimage.php

<?php
sleep(30);
header('Cache-Control: no-cache, must-revalidate');
header('Location: data:image/gif;base64,R0lGODlhAQABAJAAAMjIyAAAACwAAAAAAQABAAACAgQBADs%3D');
?>