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

Based on the discussion in
w3c/csswg-drafts#10806 and also in
w3c/csswg-drafts#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}

css/css-shadow-parts/pseudo-classes-after-part.html

@@ -10,6 +10,8 @@
 
 // Test that non-structural pseudo-classes are allowed after ::part().
 test_valid_selector("::part(mypart):active");
+test_valid_selector("::part(mypart):active-view-transition");
+test_valid_selector("::part(mypart):active-view-transition-type(mytype)");
 test_valid_selector("::part(mypart):any-link");
 test_valid_selector("::part(mypart):autofill");
 test_valid_selector("::part(mypart):checked");
@@ -68,16 +70,13 @@
 test_invalid_selector("::part(mypart):nth-of-type(1)");
 test_invalid_selector("::part(mypart):only-child");
 test_invalid_selector("::part(mypart):only-of-type");
-// These are structural because they only match the root element.
-test_invalid_selector("::part(mypart):active-view-transition");
-test_invalid_selector("::part(mypart):active-view-transition-type(mytype)");
 
 // Test that pseudo-classes that match based on tree information are not allowed after ::part.
 test_invalid_selector("::part(mypart):host");
 test_invalid_selector("::part(mypart):host-context(:hover)");
 test_invalid_selector("::part(mypart):has(p)");
 test_invalid_selector("::part(mypart):has(:hover)");
-test_invalid_selector("::part(mypart):scope)");
+test_invalid_selector("::part(mypart):scope");
 
 // Test that :not() applies appropriate constraints for being after ::part(),
 // but that :is() and :where() (which accept <forgiving-selector-list>) do