Add LayoutTest for ServiceWorker's sandbox iframe handling.

LayoutTests/http/tests/serviceworker/chromium/resources/sandboxed-iframe-fetch-event-worker.js

+var requests = [];
+
+self.addEventListener('message', function(event) {
+    self.clients.matchAll()
+      .then(function(clients) {
+          var client_urls = [];
+          for(var client of clients){
+            client_urls.push(client.url);
+          }
+          client_urls = client_urls.sort();
+          event.data.port.postMessage(
+              {clients: client_urls, requests: requests});
+        });
+  });
+
+self.addEventListener('fetch', function(event) {
+    requests.push(event.request.url);
+    event.respondWith(fetch(event.request));
+  });