Convert #pragma allow_unsafe_* to PA_UNSAFE_TODO [4/4] (apple/mac)

This removes the #ifdef UNSAFE_BUFFERS_BUILD blocks from
base/allocator/partition_allocator/src and wraps the newly exposed
unsafe buffer accesses with PA_UNSAFE_TODO() macros to allow the
build to succeed.

Bug: 40284755
Change-Id: I2b8c2783aab09cf4245c0181f0198fe62c8ec979
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7691015
Reviewed-by: Stephen Nusko <[email protected]>
Commit-Queue: Arthur Sonzogni <[email protected]>
Auto-Submit: Arthur Sonzogni <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1604039}
NOKEYCHECK=True
GitOrigin-RevId: 3666a0bb9b7f1c12eb0db80f50531ffd3856a6dc
15 files changed