Import w3c tests for the service workers

third_party/WebKit/LayoutTests/imported/wpt/service-workers/service-worker/resources/fetch-response-xhr-worker.js

 self.addEventListener('fetch', function(event) {
     var url = event.request.url;
     if (url.indexOf('dummy?test') == -1) {
       return;
     }
     event.respondWith(new Promise(function(resolve) {
         var headers = new Headers;
         headers.append('foo', 'foo');
         headers.append('foo', 'bar');
         resolve(new Response('hello world', {'headers': headers}));
       }));
   });