[animation-trigger] Fix crash when trigger source has ::selection style

In the linked clusterfuzz bug, Blink would perform
blink::Element::RecalcHighlightStyles which ended up invoking
blink::CSSAnimations::UpdateNamedTriggers again, except without
CSSAnimationData, i.e. StyleBuilder().Animations() is null, even though
in an earlier pass while styling the same element it was not.

This resulted in clobbering the trigger that had already been correctly
created, replacing it with a null pointer.

Bug: 451477493, 390314945
Change-Id: Ib192410e0b408e6af8fe38fcfa54923d3ebf2151
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7063693
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: David Awogbemila <awogbemila@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1533669}