CSS: WPT for scroll-padding computed value

Computed value is per side, either the keyword auto or a computed
<length-percentage> value.

https://drafts.csswg.org/css-scroll-snap-1/#propdef-scroll-padding

Change-Id: Id3200ae8a8af9a2e8ee87e675d14f3a6965ea841
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619454
Auto-Submit: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661303}
9 files changed