Spanify `StylePropertyShorthand::properties_`

This patch spanifies the storage only, without changing the
public interfaces. Interface changes are planned in future
patches.

This patch changes the `unsigned length_` to `size_t` in
`base::span`. When `size_t` is larger than `unsigned`, it
increases the size of `StylePropertyShorthand`. Note, every
shorthand property has one static instance of this class.

This patch should have no behavior changes.

Bug: 351564777
Change-Id: I0125a2f1a29e00cd87fe2db20a6734262bcd2816
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5759001
Commit-Queue: Kent Tamura <tkent@chromium.org>
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1337092}