[CSS] Support ::part with ::before/::after

This enables it for ::before, ::after and ::placeholder and adds a
test for all 3 but for reasons I haven't dug into yet, ::placeholder
doesn't work with ::part, so this includes a failing expectation.

Bug: 921908
Change-Id: I60a859ede98f8284cb3a1e44c81a33230b490239
Reviewed-on: https://chromium-review.googlesource.com/c/1414536
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623198}
4 files changed