Fix some PA_UNSAFE_TODOs in //base/allocator/partition_allocator

Use std::array instead of native array.

Bug: 40285824
Change-Id: I652094ad6a36c77bd9cfdd56e8afa772096f581a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7547665
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Commit-Queue: YISI YU <hellofroser@gmail.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Auto-Submit: YISI YU <hellofroser@gmail.com>
Cr-Commit-Position: refs/heads/main@{#1580490}
NOKEYCHECK=True
GitOrigin-RevId: 79ff5fc7f3bf1421ce631e6eb3aabf13d4510f4a