Improve clarity of the notification permission test for detached contexts

Rename variables to be clearer in their intention, use a switch
statement instead of an if/else if/else blocks and wrap the whole thing
as a step function to properly catch failing asserts.

Bug: 
Change-Id: I89c92d7a5071728bfdd8c9827f08ae3af94377db
Reviewed-on: https://chromium-review.googlesource.com/809425Reviewed-by: Anita Woodruff <awdf@chromium.org>
Commit-Queue: Anita Woodruff <awdf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522076}

third_party/WebKit/LayoutTests/http/tests/notifications/request-permission-detached-context.html

@@ -15,26 +15,36 @@
           testRunner.setCanOpenWindows();
       }
 
-      async_test(function (test) {
-          var win = window.open('resources/window-permission-detached-context.html'),
-              notificationObj = null;
-
-          window.addEventListener('message', function (event) {
-              if (event.data == 'opened') {
-                  assert_own_property(win, 'Notification');
-                  notificationObj = win.Notification;
-                  win.close();
-
-              } else if (event.data == 'closed') {
-                  assert_equals(notificationObj.permission, 'denied');
-                  notificationObj.requestPermission(function () {});
-
-                  test.done();
-              } else {
-                  assert_unreached('Unrecognized message from the opened window.');
-              }
-          });
+      async_test(test => {
+          const remoteWindow = window.open('resources/window-permission-detached-context.html');
+          let remoteNotificationObj = null;
+
+          window.addEventListener('message', test.step_func(event => {
+              switch (event.data) {
+                  case 'opened':
+                      assert_equals(remoteNotificationObj, null);
+                      remoteNotificationObj = remoteWindow.Notification;
+
+                      remoteWindow.close();
+                      break;
+
+                  case 'closed':
+                      assert_not_equals(remoteNotificationObj, null);
 
+                      // Permission is always set to denied for detached objects.
+                      assert_equals(remoteNotificationObj.permission, 'denied');
+
+                      // The following call should not throw an exception.
+                      remoteNotificationObj.requestPermission(function () {});
+
+                      test.done();
+                      break;
+
+                  default:
+                      assert_unreached('Unrecognized message from the opened window.');
+                      break;
+              }
+          }));
       }, 'Requesting notification permission in a detached context should not crash.');
     </script>
   </body>