[PA] Adjust GWP-ASan BRP flags

This change is mostly a no-op, plumbing GWP-ASan support in the
PartitionAlloc build recipe directly to BRP's "put refcount in previous
slot" support. An assert in `partition_alloc.gni` (the converse of the
assert removed here) already provides the appropriate enforcement.

Note that while "put refcount in previous slot" is conceptually a
perfect proxy for BRP enablement, we do not wish to remove it right now,
as we may elect to implement an alternate mode for refcount position at
some later time. In any case, it's already part of our public build API,
so we can save ourselves some churn here.

Bug: 1371729
Change-Id: I396e4203806e9d64730e1b01ec74258c271e03ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4065641
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/main@{#1077369}