Upstream service worker `fetch` tests to WPT

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/fetch-response-xhr-iframe.https.html

 <script src="/common/get-host-info.sub.js"></script>
 <script src="test-helpers.sub.js?pipe=sub"></script>
 <script>
 var host_info = get_host_info();
 
 function xhr_send(method, data) {
   return new Promise(function(resolve, reject) {
       var xhr = new XMLHttpRequest();
       xhr.onload = function() {
         resolve(xhr);
       };
       xhr.onerror = function() {
         reject('XHR should succeed.');
       };
       xhr.responseType = 'text';
       xhr.open(method, './dummy?test', true);
       xhr.send(data);
     });
 }
 
 function coalesce_headers_test() {
   return xhr_send('POST', 'test string')
   .then(function(xhr) {
       window.parent.postMessage({results: xhr.getResponseHeader('foo')},
                                 host_info['HTTPS_ORIGIN']);
+
+      return new Promise(function(resolve) {
+          window.addEventListener('message', function handle(evt) {
+              if (event.data !== 'ACK') {
+                return;
+              }
+
+              window.removeEventListener('message', handle);
+              resolve();
+            });
+        });
     });
 }
 
 window.addEventListener('message', function(evt) {
-    var port = evt.ports[0];
+    var port;
+
+    if (event.data !== 'START') {
+      return;
+    }
+
+    port = evt.ports[0];
+
     coalesce_headers_test()
       .then(function() { port.postMessage({results: 'finish'}); })
       .catch(function(e) { port.postMessage({results: 'failure:' + e}); });
   });
 </script>