Service Worker: Make ServiceWorkerRegistration.update() return a promise. (Blink Layout 3/3)

LayoutTests/http/tests/serviceworker/resources/update-worker.php

 <?php
-// Force the browser to cache this script. update() should always bypass this
-// cache and fetch a new version.
-header('Cache-Control: max-age=86400');
+if(!isset($_COOKIE['mode']))
+  $mode = 'init'; // Set mode to 'init' for initial fetch.
+else
+  $mode = $_COOKIE['mode']; // $_COOKIE['mode'] is either 'normal' or 'error'.
 
+// no-cache itself to ensure the user agent finds a new version for each update.
+header("Cache-Control: no-cache, must-revalidate");
+header("Pragma: no-cache");
+
+if ($mode == 'init') {
+  // Set a normal mimetype.
+  // Set cookie value to 'normal' so the next fetch will work in 'normal' mode.
+  header('Content-Type:application/javascript');
+  setcookie('mode', 'normal');
+} else if ($mode == 'normal') {
+  // Set a normal mimetype.
+  // Set cookie value to 'error' so the next fetch will work in 'error' mode.
+  header('Content-Type:application/javascript');
+  setcookie('mode', 'error');
+} else if ($mode == 'error') {
+  // Set a disallowed mimetype.
+  // Unset and delete cookie to clean up the test setting.
+  header('Content-Type:text/html');
+  unset($_COOKIE['mode']);
+  setcookie('mode', '', time() - 3600);
+}
 // Return a different script for each access.
-header('Content-Type:application/javascript');
-echo '// ' . microtime()
+echo '// ' . microtime();
 ?>