Upstream foreign fetch tests.

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/foreign-fetch-helper-worker.js

+importScripts('get-host-info.sub.js');
+const host_info = get_host_info();
+
+self.onfetch = e => {
+  const remote_url = host_info.HTTPS_REMOTE_ORIGIN +
+                     new URL('./', location).pathname + 'simple.txt?basic_sw';
+  e.respondWith(fetch(remote_url));
+};