Refactor ServiceWorker tests for inclusion in WPT

third_party/WebKit/LayoutTests/http/tests/serviceworker/navigation-preload/resources/request-headers-worker.js

-self.addEventListener('activate', event => {
-    event.waitUntil(
-        Promise.all[
-            self.registration.navigationPreload.enable(),
-            self.registration.navigationPreload.setHeaderValue('hello')]);
-  });
-
-self.addEventListener('fetch', event => {
-    event.respondWith(event.preloadResponse);
-  });