[partition_alloc] Add alternate bucket distribution for PartitionAlloc

This should not change any behaviour, but merely allow us to run a Finch
experiment comparing the two distribution.

Bug: None
Change-Id: Ifaac5a1cbcf5d21ce44fa626f57b91a3ed4d7a69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3272015


Reviewed-by: Benoit Lize <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Thiabaud Engelbrecht <thiabaud@google.com>
Cr-Commit-Position: refs/heads/main@{#965816}