Test for register canceling an unregister.

LayoutTests/http/tests/serviceworker/resources/test-helpers.js

 // Adapter for testharness.js-style tests with Service Workers
 
 function service_worker_unregister_and_register(test, url, scope) {
     var options = scope ? { scope: scope } : {};
     return navigator.serviceWorker.unregister(scope).then(
         test.step_func(function() {
             return navigator.serviceWorker.register(url, options);
         }),
         unreached_rejection(test, 'Unregister should not fail')
     ).then(test.step_func(function(worker) {
           return Promise.resolve(worker);
         }),
         unreached_rejection(test, 'Registration should not fail')
     );
 }
 
 function service_worker_unregister_and_done(test, scope) {
     return navigator.serviceWorker.unregister(scope).then(
         test.done.bind(test),
         unreached_rejection(test, 'Unregister should not fail'));
 }
 
 // Rejection-specific helper that provides more details
 function unreached_rejection(test, prefix) {
     return test.step_func(function(error) {
-        var reason = error.name ? error.name : error;
-        var prefix = prefix ? prefix : "unexpected rejection";
-        assert_unreached(prefix + ': ' + reason);
+        var reason = error.message || error.name || error;
+        var error_prefix = prefix || 'unexpected rejection';
+        assert_unreached(error_prefix + ': ' + reason);
     });
 }
 
 // FIXME: Clean up the iframe when the test completes.
 function with_iframe(url, f) {
     return new Promise(function(resolve, reject) {
         var frame = document.createElement('iframe');
         frame.src = url;
         frame.onload = function() {
             if (f) {
               f(frame);
             }
             resolve(frame);
         };
         document.body.appendChild(frame);
     });
 }
 
+function unload_iframe(iframe) {
+  var saw_unload = new Promise(function(resolve) {
+      iframe.contentWindow.addEventListener('unload', function() {
+          resolve();
+        });
+    });
+  iframe.src = '';
+  iframe.remove();
+  return saw_unload;
+}
+
 function normalizeURL(url) {
   return new URL(url, document.location).toString().replace(/#.*$/, '');
 }
 
 function wait_for_state(test, worker, state) {
     return new Promise(test.step_func(function(resolve, reject) {
         worker.addEventListener('statechange', test.step_func(function() {
             if (worker.state === state)
                 resolve(state);
         }));
@@ skipping 72 matching lines @@
         HTTP_ORIGIN: 'http://' + ORIGINAL_HOST + ':' + HTTP_PORT,
         HTTPS_ORIGIN: 'https://' + ORIGINAL_HOST + ':' + HTTPS_PORT,
         HTTP_REMOTE_ORIGIN: 'http://' + REMOTE_HOST + ':' + HTTP_PORT,
         HTTPS_REMOTE_ORIGIN: 'https://' + REMOTE_HOST + ':' + HTTPS_PORT
     };
 }
 
 function base_path() {
     return location.pathname.replace(/\/[^\/]*$/, '/');
 }