Reland "[PA] Make PartitionRoot a class"

This reverts commit e260315ba2f64b827705e4355891f92920ce3e7d.

Reason for revert: Failure fixed; see diff from PS1

Original change's description:
> Revert "[PA] Make PartitionRoot a class"
>
> This reverts commit 51c088b1846e62d2709c62139ecb402cf476ed46.
>
> Reason for revert: Change is suspected to cause build failures.
> https://ci.chromium.org/ui/p/chromium/builders/ci/linux-extended-tracing-rel/100068/blamelist
>
> Original change's description:
> > [PA] Make PartitionRoot a class
> >
> > `PartitionRoot` is one of the most complex type within PartitionAlloc,
> > with non-trivial constructors and a destructor. It is not simple enough
> > to be a struct, hence changing to a class.
> >
> > This change also introduces trailing underscores to its member variable
> > names.
> >
> > Fuchsia-Binary-Size: Pure refactoring.
> > Change-Id: Id344859356522e7bec6b9207b9cc94819a67eabe
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7364660
> > Reviewed-by: Takashi Sakamoto <tasak@google.com>
> > Commit-Queue: Mikihito Matsuura <mikt@google.com>
> > Cr-Commit-Position: refs/heads/main@{#1568957}
>
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Change-Id: I22c22203ed6f26fdc7fac6fd662d7b7de3c82a82
> Bug: 7364660
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7461033
> Reviewed-by: Matthias Körber <koerber@google.com>
> Owners-Override: Matthias Körber <koerber@google.com>
> Auto-Submit: Matthias Körber <koerber@google.com>
> Commit-Queue: Matthias Körber <koerber@google.com>
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Cr-Commit-Position: refs/heads/main@{#1568964}

Cq-Include-Trybots: luci.chromium.try:linux-extended-tracing-rel
Bug: 475729821
Change-Id: I08bfaa4bbfcce9586177d7116ab6c8cdc8b8b48b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7501136
Commit-Queue: Mikihito Matsuura <mikt@google.com>
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/main@{#1575048}
NOKEYCHECK=True
GitOrigin-RevId: 4c9f31db6b9d8d8ca9d3036be16bd00425e5a2cb