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 i of clients){

[nhiroki, 2015/06/26 01:01:36]

super-nit: "c" would be more natural that "i"

[horo, 2015/06/26 03:59:10]

Done.

+            client_urls.push(i.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));
+  });