Intent to prototype: Standard :autofill pseudo-class, :-webkit-autofill alias.

[Emilio Cobos Álvarez]

Summary:

I plan to prototype a standard :autofill pseudo-class, for two reasons

* We have an internal :-moz-autofill pseudo-class, and webkit-based
browsers have a publicly-exposed :-webkit-autofill counter-part.
* We have a fair amount of compat reports of people forgetting
:-webkit-autofill is not standard and making unrelated CSS not apply to
Firefox.

Thus, I think it'd be nice to have an unprefixed pseudo-class authors
can use, and parse the :-webkit- prefix (unfortunately) for compat.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1475316

Standards: https://github.com/whatwg/html/issues/6181 and
https://github.com/w3c/csswg-drafts/issues/5775 for the relevant spec
discussion where I hope to get consensus.

Pref: layout.css.autofill.enabled.

Devtools Bug: I don't think devtools needs particular support. We could
add an :autofill checkbox to the pseudo-classes submenu, please file and
ni? me if you think it'd be useful.

Other browsers: Both WebKit and Blink ship for a long time prefixed
(:-webkit-autofill)

web-platform-tests: I'll add parsing tests once the spec discussion is
done. We can't do much more because actual autofill behavior is not
particularly standard.