[carousel] Update marker group scroll snap areas when they change.

Replacing the innards of a scroll marker group when it gets laid out by
a sibling (the scroller) requires more rocket surgery than what we had.
Just update the propagated_data_ member, which happens to contain any
snap areas (and other goodies too, which should benefit from being
carried over properly).

Move MutableForOofFragmentation::Merge() into the .cc file, since it's a
grown-up function now.

Bug: 402354445
Change-Id: I1893d5d0390c0f113d7ba648a8ba68c842e02005
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6352056
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Daniil Sakhapov <sakhapov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1432592}