Service Worker: Correct Web IDL of FetchEvent.respondWith method.

LayoutTests/http/tests/serviceworker/resources/fetch-event-respond-with-argument-worker.js

+self.addEventListener('fetch', function(event) {
+    var testcase = new URL(event.request.url).search;
+    switch (testcase) {
+    case '?response-object':
+      event.respondWith(new Response('body'));
+      break;
+    case '?response-promise-object':
+      event.respondWith(Promise.resolve(new Response('body')));
+      break;
+    case '?other-value':
+      event.respondWith(new Object());
+      break;
+    }
+  });