Due to missing locks, there is a race between AllocatePagesAt (or
ReserveForSharedMemoryMapping) and other functions that modify
std::sets in RegionAllocator (e.g. AllocatePages or ReleasePages).

The CL adds locks to AllocatePagesAt and ReserveForSharedMemoryMapping.

Bug: chromium:1232067
Change-Id: I0ec503ab1ab432952ea067eb916299ea88566879
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3056985
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75981}