[PA] Namespace migration (34.1 of N)

This CL removes some `base` aliases from the `partition_alloc_forward`
header.

This CL is being split off from crrev.com/c/3573827 because of an
inexplicable test failure on the win10_chromium_x64_rel_ng bot:

[  FAILED  ] rendering.desktop/filter_terrain_svg higher average
frame_times(21.664) compared to upper limit (19.173)

Bug: 1288247
Change-Id: Ib2030d1a60f53362d6a22aa98579201e543bbfee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3598394


Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#995411}