[html] Avoid race condition in test

html/browsers/the-window-object/support/noopener-target.html

@@ -4,5 +4,12 @@
   var channel = new BroadcastChannel(channelName);
   channel.postMessage({ name: window.name,
                         haveOpener: window.opener !== null });
-  window.close();
+
+  // Because messages are not delivered synchronously and because closing a
+  // browsing context prompts the eventual clearing of all task sources, this
+  // document should not be closed until the opener document has confirmed
+  // receipt.
+  channel.onmessage = function() {
+    window.close();
+  };
 </script>

html/browsers/the-window-object/window-open-noopener.html

@@ -64,7 +64,10 @@
   } else {
     t.step(function() {
       var w1;
-      this.add_cleanup(function() { w1.close(); });
+      this.add_cleanup(function() {
+        w1.close();
+        channel.postMessage(null);
+      });
 
       var windowName = this.windowName;
       var channel = new BroadcastChannel(windowName);