Disable hopelessly flaky test sandboxed-iframe-autofocus-denied.html

BUG=381451
TBR=peter@chromium.org

Review URL: https://codereview.chromium.org/321493002

git-svn-id: svn://svn.chromium.org/blink/trunk@175598 bbb929c8-8fbe-4397-9dbb-9b2b20218538