[WPT/common/security-features] Fix typo

Fix a typo introduced by
https://chromium-review.googlesource.com/c/chromium/src/+/1788876
(Upstream: https://github.com/web-platform-tests/wpt/pull/18904)

Change-Id: I9ac2d8048c14c42e18d44f643b8dfcfac47c5fd9
Bug: 1001422
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1834886Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702278}

third_party/blink/web_tests/external/wpt/common/security-features/resources/common.sub.js

@@ -1225,7 +1225,7 @@ function invokeFromIframe(subresource, sourceContextList) {
     promise = fetch(frameUrl)
       .then(r => r.text())
       .then(t => {
-          frameContnent = t;
+          frameContent = t;
           iframe = createElement("iframe", {}, document.body, true);
           return iframe.eventPromise;
         })