Nested view transitions: support view-transition-group: nearest/normal

Parse nearest/normal as keywords, and use a StyleViewTransitionGroup
struct to store the result.

Rely on a stack while capturing elements hierarchically, to
correctly resolve "view-transition-group: nearest".

Also use that stack instead of element ancestry for
computing explicit names.

Bug: 347947051
Change-Id: If06d2a6ff266013f4a46a492191c31e4a66111c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5752182
Reviewed-by: Khushal Sagar <khushalsagar@chromium.org>
Commit-Queue: Noam Rosenthal <nrosenthal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1338320}