Allow almost all other pseudo-elements after ::part().
This allows, behind the CSSPartAllowsMoreSelectorsAfter flag
(status:experimental), using almost any pseudo-element other than
::part() itself after ::part(). It does not allow ::slotted(); see link
to relevant spec issue in the code comments.
NOTE: Some of the added tests are failing due to getComputedStyle()
returning element styles for UA shadow pseudo-elements. See
crbug.com/363015176.
Bug: 40825557, 363015176
Change-Id: I85d11146c494407f2a6263b444ceed7bdd1ab4d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5823774
Commit-Queue: David Baron dbaron@chromium.org
Reviewed-by: Rune Lillesveen futhark@chromium.org
Cr-Commit-Position: refs/heads/main@{#1349865}