Intent to prototype and ship: Make ::backdrop inherit from its originating element.

[Emilio Cobos Álvarez]

Since I have memory, ::backdrop was a special pseudo-element that didn't
inherit from anywhere. This had various implications, such as custom
properties not being inherited into it, and in general just being an odd
thing (all other pseudos inherit from their originating element).

The fullscreen spec used to require this behavior, but it wasn't clear
why, see the discussion in [1].

When these pseudos were moved to CSS specs, this got fixed[2]. So I'm
fixing up the tests and implementing the new behavior.

The new behavior is much better for authors, and I don't expect
meaningful compat fallout, since:

* ::backdrop is a fairly limited pseudo-element (doesn't have text /
content that uses inherited properties by default).
* Using inheritance was kinda silly, if you were using `inherit`, you
can use `initial` now.

The main thing that could break is if somebody, for some reason, would
make the <dialog> or fullscreen-element invisible via `visibility`, and
expect a ::backdrop to be visible. Some tests did this (to test backdrop
rendering), but it seems something unexpected in practice, which could
also be fixed via a one-liner UA stylesheet rule if needed.

Cheers,

-- Emilio

[1]: https://github.com/whatwg/fullscreen/issues/124
[2]: https://drafts.csswg.org/css-position-4/#backdrop