Intent to prototype and ship: align-content on block containers

[Emilio Cobos Álvarez]

Summary: In bug 1684236 I landed support for align-content on block
containers. This allows trivial vertical alignment in block layout.
See [1] for a good blog post on the matter.

I plan to enable it by default in bug 1882853, for this release, if
nothing goes wrong.

This feature has some compat risk because this property used to have no
effect at all on blocks, but it seems Blink is on its way to ship it in
their next release, so risk should be manageable (and it's behind a pref
anyways so it's easy to turn off as needed).

Huge thanks to Elika (fantasai) for providing the initial version of the
patch and a lot of tests!

Bug: https://bugzil.la/1684236 / https://bugzil.la/1882853
Specification: https://drafts.csswg.org/css-align/#align-justify-content
Standards Body: CSS Working Group
Platform coverage: All
Preference: layout.css.align-content.blocks.enabled
DevTools bug: Sebastian was kind enough to file
https://bugzil.la/1882741, thanks!
Link to standards-position discussion: None (fairly uncontroversial
extension to css-align, IMO, but happy to file one).
Other browsers:

* Blink: Will ship in 123[2]
* WebKit: Has an implementation[3], though not sure when it'll ship.

web-platform-tests: https://wpt.fyi/results/css/css-align/blocks

(The two tests that we fail had a wrong reference, and that's fixed on
autoland already)

Let me know if there's any concern here or what not.

Cheers,

-- Emilio

[1]: https://developer.chrome.com/blog/align-content
[2]: https://chromestatus.com/feature/5159040328138752
[3]: https://github.com/WebKit/WebKit/pull/21522