[PA] Move README, OWNERS, DEPS, BUILD.gn to top-level dir.

This is a follow-up to:
https://chromium-review.googlesource.com/c/chromium/src/+/4812338

The previous patch moved the whole PA directory into
"src/partition_alloc" without putting much thinking about individual
files.

This follow-up moves the README, OWNERS, DEPS, BUILD.gn file into the
top-level directory.

Bug: chromium:1467773
Change-Id: Ie9f457fe957bbf27779892eece606eb220d50cb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4934632
Owners-Override: danakj <danakj@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1210356}
NOKEYCHECK=True
GitOrigin-RevId: c8ca46524226127e51531c08bab01d049d4a3083
2 files changed