[scroll-animations-1] Make timeline name scoping last-in-tree-order

[andruud]

See #12581, #13364.

[andruud]

Hmm, actually, I'm not sure this works correctly.

<div id=t1 style="scroll-timeline: --t; animation-timeline:--t">
  <div id=t2 style="scroll-timeline: --t">
  </div>
</div>

This PR would cause animation-timeline:--t to find --t on #t2. I'll try again next week.