Bug 1849737 [wpt PR 41584] - Fix influence of <slot> on dir=auto computation of its...

Bug 1849737 [wpt PR 41584] - Fix influence of <slot> on dir=auto computation of its shadow tree ancestors., a=testonly

Automatic update from web-platform-tests
Fix influence of <slot> on dir=auto computation of its shadow tree ancestors.

This change treats a <slot> element as being a strong character, of its
resolved directionality, when resolving dir=auto on its shadow tree
ancestor.

This is behind the RuntimeEnabledFeatures::CSSPseudoDirEnabled() flag
because we're hoping to ship that feature soon and it makes sense to
ship related changes to direction handling all at once rather than
piecemeal.

This is based on the proposed behavior described in:
https://github.com/whatwg/html/issues/3699#issuecomment-951423468
which is in the process of being specified in:
https://github.com/whatwg/html/pull/9166
https://github.com/whatwg/html/pull/9452
https://github.com/whatwg/html/pull/9554

This fixes the failures of:
external/wpt/shadow-dom/directionality/dir-shadow-30.html
external/wpt/shadow-dom/directionality/dir-shadow-34.html
in the still-unlanded WPT PR at
https://github.com/web-platform-tests/wpt/pull/29820

This also changes the existing WPT
html/dom/elements/global-attributes/dir-slots-directionality.tentative.html
in the following ways:
 * split the test into separate test() functions to get separate results
 * add a sixth test testing <slot dir=auto></slot>
 * add tests of the :dir() selector for each test (where Chromium fails
   this test for test 1)
 * change the expected result of the fourth test to match this code
   change and the proposed specification

Bug: 576815
Change-Id: I83551e9bc5807109c5318bace486cfc93fc25bbb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4800366
Reviewed-by: Di Zhang <dizhangg@chromium.org>
Commit-Queue: David Baron <dbaron@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1186743}

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wpt-commits: d0900810fbec1d01a2c86fc9c59333b8c84c0fc3
wpt-pr: 41584