The serializer collects objects in iteration order, not in allocation
order. This means that the deserializer will put these objects in
iteration order onto the reserved pages as well. There is no guarantee
that objects that were on the first page will end up on the first page
after deserialization.

Until now we got lucky, since we only ever need one space per page for
the default snapshot. For roots, the iteration order and allocation
order also do not differ enough to cause any issue for immortal
immovable root objects. These objects need to stay on the first page of
its allocated space to not move.

However, let's make sure it stays this way, and we realize soon enough
if this assumption does not hold.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1675553002

Cr-Commit-Position: refs/heads/master@{#33810}