[@scope] Allow inserting CSSNestedDeclarations rule in top-level @scope
This was previously only allowed when @scope was a nested group rule,
but as of CL:6020075 it's allowed top level as well.
Bug: 351045927
Change-Id: Ic6d60f745c433a759f52b3e0e0e6903fc0a57f38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6039155
Reviewed-by: Steinar H Gunderson sesse@chromium.org
Commit-Queue: Anders Hartvoll Ruud andruud@chromium.org
Cr-Commit-Position: refs/heads/main@{#1388174}