Add layout tests for Service Worker fallbacked subresource requests.

LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-worker.js

+// TODO(horo): Service worker can be killed at some point during the test. So we
+// should use storage API instead of this global variable.
 var requests = [];
 
 self.addEventListener('message', function(event) {
     event.data.port.postMessage({requests: requests});
+    requests = [];
   });
 
 self.addEventListener('fetch', function(event) {
     requests.push({
         url: event.request.url,
         mode: event.request.mode
       });
   });