[GridNG] Introduce NGGridLayoutSubtree to NGConstraintSpace

1. Introducing `NGGridLayoutTree`, a tree structure that holds the final
   track collections for the entire grid tree. In order to inherit this
   structure to subgrids via `NGConstraintSpace` we also introduce
   `NGGridLayoutSubtree`, which is a class that has the ability to
   access the layout tree and iterate over its nodes.

2. Removing calls to `ComputeMinMaxSizes` and `Layout` for subgrids in
   favor of an upcoming method that will compute the used track sizes of
   its standalone axis (if any). As a result of this, sizing and layout
   of subgrids will be incorrect until we introduce the new method.

Bug: 618969
Change-Id: I26c9b2e0d49d32517b30bc710e046f0a0ee8fd07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4195814


Commit-Queue: Ethan Jimenez <ethavar@microsoft.com>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118923}