Refactor ServiceWorker tests for inclusion in WPT

third_party/WebKit/LayoutTests/http/tests/serviceworker/navigation-preload/resources/broken-chunked-encoding-worker.js

-self.addEventListener('activate', event => {
-    event.waitUntil(
-        self.registration.navigationPreload.enable());
-  });
-
-self.addEventListener('fetch', event => {
-    event.respondWith(event.preloadResponse
-      .then(
-        _ => new Response('Fail: got a response'),
-        _ => new Response('Done')));
-  });