Test for register canceling an unregister.

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

Index: LayoutTests/http/tests/serviceworker/resources/test-helpers.js
diff --git a/LayoutTests/http/tests/serviceworker/resources/test-helpers.js b/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
index 5a72d31ca1bac7a061c90a030c25e6311e75204e..51d8ebbb0a4f05b4233d4301cd7c5dbab359481d 100644
--- a/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
+++ b/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
@@ -23,9 +23,9 @@ function service_worker_unregister_and_done(test, scope) {
 // 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);
     });
 }
 
@@ -44,6 +44,17 @@ function with_iframe(url, f) {
     });
 }
 
+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(/#.*$/, '');
 }