Chrome Version: 60.0.3078.0

What steps will reproduce the problem?

Open a page with the following content:

<iframe sandbox="allow-scripts"
        src="data:text/html,<script>
               window.addEventListener('beforeunload', function (event) {
                   event.returnValue = 'MESSAGE';
               });
               window.addEventListener('load', function() {
                  location = 'https://duckduckgo.com';
               });
            </script>"></iframe>

What is the expected result?

Modal dialogs should be blocked per https://html.spec.whatwg.org/multipage/origin.html#sandboxed-modals-flag

What happens instead?

A modal dialog is opened, asking whether we want to leave the page