Upstream service worker `fetch` tests to WPT

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

Index: third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/fetch-response-xhr-iframe.https.html
diff --git a/third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/fetch-response-xhr-iframe.https.html b/third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/fetch-response-xhr-iframe.https.html
index 08b887b7de662e9c9b1b2542bbb359742c914212..1414596c16e324e9160643579945cb32166076be 100644
--- a/third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/fetch-response-xhr-iframe.https.html
+++ b/third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/fetch-response-xhr-iframe.https.html
@@ -23,11 +23,29 @@ function coalesce_headers_test() {
   .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}); });