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_test(url, description) {
     var t = async_test(description);
     t.step(function() {
         var scope = 'nonexistent';
         service_worker_unregister_and_register(t, url, scope).then(t.step_func(onRegistered));
 
         function onRegistered(worker) {
             var messageChannel = new MessageChannel();
@@ skipping 45 matching lines @@
         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();
+    iframe = null;
+    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);
         }));
     }));
 }