[LayoutNG] Lay out OOFs in inner multicol during nested fragmentation

This CL is the first of several to add support for OOF layout in
a nested fragmentation context. To allow the OOF elements to get added
to the inner multicol rather than the outermost, a new builder
and constraint space are created for the inner multicol. Doing so also
allows us to re-use the existing OOF code with minimal effort.

Once the OOFs have been laid out and the new column fragments are
created, we need to replace the associated column fragments inside
the existing multicol fragment (instead of inside the builder itself).
In order to accomplish this, I've exposed a mutable accessor to the
children of a physical fragment.

A simple unit test was added to illustrate what is currently
working. It appears that the static position is not quite right yet,
and there is another related unit test that is hitting a DCHECK.
These will be addressed in follow-up changes, along with a few
other added TODOs (as well as additional testing).

Bug: 1079031
Change-Id: Ia529fd13b574797888deb19f7c5315eea6d9c906
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2674410
Commit-Queue: Alison Maher <almaher@microsoft.com>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#852223}