[css-properties-values-api] List syntaxes should accept one or more items.

Currently we accept zero items, which is wrong.

R=futhark@chromium.org

Bug: 641877
Change-Id: I8acdc4c5c7ba54a71a478e7eed79223271e1e92d
Reviewed-on: https://chromium-review.googlesource.com/c/1290889
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601128}
3 files changed