[BRP] Allow extras size to be controlled via Finch

Due to Out of bounds writes we are now detecting on iOS when switching
Malloc for BRP we want to try to avoid corruption of BRP datastructures
by allowing extra padding to allow iOS code to do the OOB in a harmless
area. This isn't ideal but should allow us to launch the protections of
BRP and investigate the OOB writes separately once we implement
detection.

Bug: 371135823
Change-Id: Iae3393afd5ce4d97e23e942205df54a3a5cd1648
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6074348
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Commit-Queue: Mikihito Matsuura <mikt@google.com>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1397744}