Part 6: Fix bug in wpt test

It appears the about-blank.https.sub.html test incorrectly was resolving the
`createBlankIframe` test with the load event object, rather than with the
created iframe, which led to an exception being thrown when accessing the
`contentDocument` property.

This patch makes returning the iframe explicit, fixing that failure.

Differential Revision: https://phabricator.services.mozilla.com/D236975

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1665474
gecko-commit: dafc01f02f5be174a76db78acdca35da27b52dc2
gecko-reviewers: farre

Author: mystor

html/browsers/origin/origin-keyed-agent-clusters/about-blank.https.sub.html

@@ -52,12 +52,13 @@
   await setBothDocumentDomains(iframe.contentWindow);
 }
 
-function createBlankIframe() {
+async function createBlankIframe() {
   const iframe = document.createElement("iframe");
   const promise = new Promise(resolve => {
     iframe.addEventListener("load", resolve);
   });
   document.body.append(iframe);
-  return promise;
+  await promise;
+  return iframe;
 }
 </script>