Allow ::highlight pseudo-styles in the MPC.

This strongly reduces RAM usage when there are custom highlights
on universal selectors (or near-universal selectors).

There is still a fair amount of GC churn for highlight pseudo-elements
that don't have a parent (i.e. a highlight higher up the tree;
this case is much more common than for normal elements, of which only
the root element lacks a parent), since we create an initial style with
a Font on it as parent, only to overwrite that Font later.

Bug: 446885272
Change-Id: I0ce7bba6d2a34b86f47f2f431d5f5d56c0f80126
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7008201
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1526770}