Expose and check origin of request in response for foreign fetch.

third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/foreign-fetch-helpers.js

+// Common helper functions for foreign fetch tests.
+
+// Installs a service worker on a different origin. Both |worker| and |scope|
+// are resolved relative to the /serviceworker/resources/ directory on a
+// remote origin.
+function install_cross_origin_worker(t, worker, scope) {
+  return with_iframe(get_host_info().HTTPS_REMOTE_ORIGIN +
+                     '/serviceworker/resources/install-worker-helper.html')
+    .then(frame => new Promise((resolve, reject) => {
+        var channel = new MessageChannel();
+        frame.contentWindow.postMessage({worker: worker,
+                                         options: {scope: scope},
+                                         port: channel.port1},
+                                        '*', [channel.port1]);
+        channel.port2.onmessage = reply => {
+            if (reply.data == 'success') resolve();
+            else reject(reply.data);
+          };
+      }));
+}