Support ::placeholder after ::slotted().

Tree-abiding pseudo elements are allowed after ::slotted() according to
css-scoping-1. According to css-pseudo-4 they are ::before, ::after,
::marker, and ::placeholder. We already allowed ::before and ::after. We
do not support ::marker. This CL allows ::placeholder.

TEST=external/wpt/css/css-scoping/slotted-parsing.html
TEST=external/wpt/css/css-scoping/slotted-placeholder.html

Bug: 902518
Change-Id: I963fa93e68c8fbd33fc8090e930be9a00f3c9bd4
Reviewed-on: https://chromium-review.googlesource.com/c/1323049
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606791}
8 files changed