[carousel] Implement layout tree attachment for ::scroll-marker

::scroll-marker pseudo elements act as `navigation dots` of carousel
(e.g. dots for choosing image in gallery).
They are flowed in ancestor's scroll container ::scroll-markers pseudo.
For now the layout boxes are not created at all, since they will be
created during layout with custom layout algo.

There is nothing to test for AX yet, since no boxes are yet created.

AX-Relnotes: n/a.
Bug: 332396355
Change-Id: Id65426492ced6ad98b97602ea4583c9c55059a75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5604506
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Daniil Sakhapov <sakhapov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1317041}