Allow :active-view-transition pseudo-classes after ::part().

Based on the discussion in https://github.com/w3c/csswg-drafts/issues/10806 and also in https://github.com/w3c/csswg-drafts/issues/10787, I think the conclusion from #10806 applies to pseudo-classes as well, and I should reverse my earlier decision to disallow the :active-view-transition pseudo-classes after ::part(), even though today it is not possible for them to match.

This makes that change behind the CSSPartAllowsMoreSelectorsAfter flag (status:experimental), which also contains other related changes.

This matches WebKit's current implementation of those pseudo-classes.

Bug: 40623497 Change-Id: I591fe30bb9e6c3a0953df318919f60dd6962d4a1 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5839421 Reviewed-by: Rune Lillesveen futhark@chromium.org Commit-Queue: David Baron dbaron@chromium.org Cr-Commit-Position: refs/heads/main@{#1351997}