Fixing a crash and adding more test cases to 'grid' shorthand

This CL fixes a crash that happened when the auto-flow values did not
exist. More test cases were added to grid-shorthand-valid.html and
grid-shorthand-invalid.html to validate the behavior.

Bug: 1028283, 1286308
Change-Id: Ic4b126298d5d7a5ff1f5290e086266126ffb664e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3379475


Reviewed-by: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Ana Sollano Kim <ansollan@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#958139}