[scroll-animations] Use longhands in high-specificity rule

The selector '#container > div' has higher specificity than a single
#id selector, so in a browser which supports setting the animation-
timeline via the animation shorthand (which we don't support yet but
will soon), the animation-timeline always becomes auto. To fix
this, use longhands instead.

This prepares for animation-timeline-in-shorthand support.

Bug: 1074052
Change-Id: I7d8a5ad2a315eb251cc0f42f5a54b1cf3b54ee45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2360374
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799185}