Fix throttling SMIL animations on <stop> elements

<stop> elements are not in the layout tree, so they would be considered
to be throttleable. Add a check for computed style as well. Factor out
the throttling checks into a helper and document them.

Fixed: 327254045
Change-Id: I9048a1a6d4af83c67c896034c3b32647e613a38b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5329732


Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Auto-Submit: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1266902}