Add support for allowing elements with display:contents to be focused.

[chromium-wpt-export-bot]

Chromestatus: https://chromestatus.com/feature/6237396851228672

This lands support for allowing elements with display:contents to be
focused, behind a feature flag that is created with status:experimental.

This is as concluded (though without a group resolution) in
w3c/csswg-drafts#2632 and as discussed a bit
in whatwg/html#1837 and proposed in
whatwg/html#9425 . Note that this diverges from
Gecko (see https://bugzil.la/1553549, https://bugzil.la/1791648, and
mozilla/standards-positions#772) and WebKit
(see https://bugs.webkit.org/show_bug.cgi?id=255149 and
WebKit/standards-positions#164).

This makes the <slot> element focusable when it has a tabindex. Note
that this does not match Gecko and WebKit, which also require that its
'display' value is changed (away from 'contents') to make it focusable.

Note that the <slot> element is still not tabbable in Chrome; this is
https://crbug.com/1428419. This also does not match Gecko and WebKit,
where it is tabbable whenever it is focusable. The added test
slot-element-tabbable.tentative.html fails both tests as a result, but
is added anyway. (Gecko and WebKit pass the display: block test but
fail the default style (display: contents) test.)

The added tests display-contents-focusable-001.html and
slot-element-focusable.tentative.html are partly fixed by this change,
but the style part is still broken due to https://crbug.com/1428420.

Bug: 1366037
Bug: 1428419, 1428420
Change-Id: I46a8ad3b6442ce07f440c8f6a07210bac305600e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3910374
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: David Baron <dbaron@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1252245}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.