This document is outdated, PartitionAlloc is used for all allocations now: https://chromium.googlesource.com/chromium/src/+/ec6b52f91a4...

It uses plain os_unfair_lock: https://chromium.googlesource.com/chromium/src.git/+/refs/he...

EDIT: It also combines os_unfair_lock with manual PAUSE spinning.