GitLab

The main partition_alloc.h header is getting unwieldy. Move locking to
its own file, in preparation for more work on the lock. No behavior
change.

Bug: 1061437
Change-Id: Ifa6314fb87a2081a6f7d32f48c1812614debb30f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390400
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803931}