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 9685b6fd6a386c28f738f179c0c2de45cfc94aca..af5509cadedfd626b1009966ca15ec314b325f64 100644
--- a/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
+++ b/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
@@ -63,6 +63,18 @@ 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();
+    iframe = null;
+    return saw_unload;
+}
+
 function normalizeURL(url) {
   return new URL(url, document.location).toString().replace(/#.*$/, '');
 }