Include child rules when adding rules to RuleSetDiff

When some style rule changes (e.g. its selector text), any inner
style rules may also effectively change due to the &/:scope selector.
We already accounted for this during RuleSet::AddFilteredRulesFrom-
OtherBucket by traversing the chain of parent rules to see if any
were marked for inclusion. However, we did this by searching for
the '&' selector specifically, which means :scope cases did not
invalidate properly.

Unlike '&', :scope does not capture its parent style rule during
parsing, and a StyleRule has no general way of reaching its parent
(we don't store that pointer). Therefore, this CL traverses the child
rules top-down instead, during RuleSetDiff::AddDiff. The end result
should be the same, except that we now also include child rules
of @scope rules.

Note: There is little point in checking if inner style rules actually
contain &/:scope in their selector or not; if omitted by the author,
we insert them implicitly.

Bug: 449542602
Change-Id: I9a42e61ede422f41e83200064f74bff6c164047a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7013292
Reviewed-by: Steinar H Gunderson <sesse@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1526740}