[scroll-animations] Modernize at-scroll-timeline-inactive-phase.html

Bug: 1317765
Change-Id: I13cb81910e48bcea805d849214ad04b04ccf6192
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3856582


Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040603}