Revert "Split PartitionAlloc headers into internal ones and public ones"

This reverts commit cd836a819c4401c7a455400ac2112d83764060c3.

Reason for revert: suspected for tree closure https://ci.chromium.org/ui/p/chromium/builders/ci/android-cast-arm-rel/32082/overview

Original change's description:
> Split PartitionAlloc headers into internal ones and public ones
>
> PS1 = baseline (copy headers into internal/)
>
> The reason why No-Try is that linux-chromeos-rel's ash_unittests fail
> because of timeout. The timeouts seemed to be caused by llvm_profiler
> (code coverage check). If this CL causes the same timeouts after
> landing, I will revert this CL.
>
> Bypass-Check-License: page_allocator_internal.h is just moved, not newly added. No need to check its license.
> No-Try: true
> Change-Id: I3481802fd95e229b1e287dabd67b4e2490d763e1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7795856
> Commit-Queue: Takashi Sakamoto <tasak@google.com>
> Reviewed-by: Lei Zhang <thestig@chromium.org>
> Reviewed-by: Sergei Glazunov <glazunov@google.com>
> Cr-Commit-Position: refs/heads/main@{#1630318}

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Ie744a441026645b582d9994878bf67426c37cbe6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7846961
Commit-Queue: Andrea Orru <andreaorru@chromium.org>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Andrea Orru <andreaorru@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1630322}
NOKEYCHECK=True
GitOrigin-RevId: 5e8bd89900b807519c27e9fe24bdf1fc0d86e2fd