dom.isFocusable should consider `audio` and `video` controls

[jeeyyy]

dom.isFocusable function to be updated to consider audio and video controls.

[marcysutton]

Focusability is pretty all over the place for those controls in browsers though, I think we need to do some testing to determine support.

[marcysutton]

To expand on what I meant by testing here: the default controls in HTML5 video and audio aren't consistently focusable, it depends on the browser. And I'm pretty sure it will also depend on whether the controls attribute is present. We need to validate whether the video and audio element controls are focusable in every browser. So far they are in Chrome, Safari and Firefox but the behavior is super inconsistent in regards to the experience and number of tab stops (thanks, user agent shadow DOM!). We should check IE11 and Edge to see if they are focusable there.

[WilcoFiers]

This is particularly relevant for a case like this:

<div aria-hidden="true">
  <video controls />
</div>

That should probably fail, whereas without the controls it shouldn't.