Scrolling content with iframe that has focusable content should not fail `scrollable-region-focusable`

[straker]

This was reported to us and after testing confirmed that a parent scrolling region which contains an iframe that has focusable content allows you to scroll the parent scrolling region. So this should not fail scrollable-region-focusable.

<div id="foo" style="margin-top: 50vh; height: 50vh; overflow-y: scroll;">
  <p style="margin-top: 100vh">Hello World</p>
  <iframe srcdoc="<button>click me</button"></iframe>
  <p style="margin-top: 100vh">Goodbye World</p>
</div>

[dbjorge]

Verified the example on stable chrome, firefox, safari