Use standards-track `allowedToPlay` if available

[gkatsev]

There's work to have a native method to know whether we're allowed to play, so, we should use that method if it's available. Mozilla said they're going to ship it in Firefox 63

whatwg/html#3617

[caiogondim]

That's amazing news to see the lib is influencing an official spec ❤️
I would say let's wait until they settle on the API and first implementation, so we can make a test with a real browser.

[gkatsev]

Yeah, definitely best to wait till firefox ships it, though, we don't have to wait for long since it's scheduled for late october.

Wanted to make sure everyone here was aware and that we were ready for the release :)

[NicolasSiver]

I was surprised from the beginning to find out that we have to do such a creative approach without any API. Browser vendors were so focused on all those user's concerns that they forgot about obvious stuff and video engineers.