Bug 1851227 [wpt PR 41766] - Make standards-track pseudo-elements override style...

Bug 1851227 [wpt PR 41766] - Make standards-track pseudo-elements override style attributes in UA shadow DOM., a=testonly

Automatic update from web-platform-tests
Make standards-track pseudo-elements override style attributes in UA shadow DOM.

This makes pseudo-elements representing elements in user-agent shadow
DOM (via Element::ShadowPseudoId()) whose names do not begin with
-webkit- or -internal- override style attributes in the user-agent
shadow DOM.  This matches developer expectations and makes sense given
https://drafts.csswg.org/css-cascade-5/#cascade-context since the
pseudo-elements are in an outer context.

The only such pseudo-elements that exist today are the experimental
::details-content and ::details-summary pseudo-elements.  (See the
DCHECK() in Element::SetShadowPseudoId().)

TODO(https://crbug.com/1478230): Devtools does not currently show the
correct cascading order since it assumes that style attributes always
win.  We should fix this before shipping this code.

Bug: 1469418, 1478230, 1479329
Change-Id: I246e6436fd1f68622f73afe2f8c709ed35ad6cd2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4823787
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: David Baron <dbaron@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1193109}

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wpt-commits: 29bbb35f49c12f778c205401ec817905a847129b
wpt-pr: 41766