This change allows clients of the RegionAllocator to provide callbacks
that are invoked when regions are split or merged.
This will later be needed on Windows when a RegionAllocator is used to
manage a placeholder mapping as these need to be split and merged (using
the VirtualFree API) as well.


Bug: chromium:1218005
Change-Id: I228b41bdb43c4a9ef0db04de9b121dea6b5f12f9
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3264287
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77903}