[@scope] Use CalculateNestingContext in setSelectorText
This fixes an issue where relative selectors in @scope rules
would get an implied '&' instead of an implied ':scope'.
Fixed: 363189649
Change-Id: Ie53414cb122d08665d93877185ca51c91263b693
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6040164
Commit-Queue: Anders Hartvoll Ruud andruud@chromium.org
Reviewed-by: Steinar H Gunderson sesse@chromium.org
Cr-Commit-Position: refs/heads/main@{#1389225}