[css-nesting] Respect nesting context when parsing @rules in ParseRule

The CSSParserImpl::ParseRule function accepts a CSSNestingType
and a parent_rule_for_nesting, but then ignores those parameters
when calling ConsumeAtRule. This does not cause any (known)
web-observable problem, but is worth fixing anyway for internal
consistency.

Added a new test util ParseNestedRule, which will primarily be used
by future CLs related to Issue 363019837.

Issue: 363019837
Change-Id: I50232cb799be6338ea4367a31d61376d4b298a94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6070883
Reviewed-by: Steinar H Gunderson <sesse@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1392300}