Make the SelectorFilter stack a bit more consistent.

Require that we never get into a situation with inconsistent
SelectorFilter parent stacks during traversal; we now explicitly
DCHECK for that instead of just pausing traversal. This also
means we need to be slightly more lenient; whereas we'd previously
accept only the flat-tree parent, we'll now also accept
the slightly more efficient (and still correct) ParentOrShadowHost().
(The flat-tree parent is used because that's the traversal order,
but it will have a few false positives in the Bloom filter.)

Unfortunately, we can still _start_ the ElementRuleCollector in an
inconsistent state; for instance, when calculating style for
pseudo-elements, when calculating style for SVG elements,
or (I think) in some shadow-root cases. Thus, we still need to
maintain the parent stack for the sole purpose of testing
can_use_fast_reject_ at the start of the traversal. We fix a few
cases related to invalidation and shadow DOM, making invalidating
those elements marginally faster.

Change-Id: I6200b734de4d7b52a9bb15682959d59815926f96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4853747
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1220183}