Move the Partition Allocator RNG to its own files.

This enables us to re-use it for other probabilistic purposes, such as
probabilistically scribbling memory on free.

Bug: 984742
Change-Id: I09267647e22a0644b2aa25464f2ec135c872c89e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1757734
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688217}
7 files changed