Delay generation of User-Agent header to URLRequestHttpJob and accept custom User-Agent from XHR/Fe…

third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/forwarding-worker.js

+let port;
+
+self.onmessage = event => {
+  const message = event.data;
+  if ('port' in message) {
+    port = message.port;
+    port.postMessage('READY');
+  }
+};
+
+self.addEventListener('fetch', event => {
+  port.postMessage('onfetch');
+
+  event.respondWith(fetch(event.request));
+});