This API allows allocating shared memory mappings inside a virtual
address space from a platform-specific handle to a shared memory object.
This will make it possible to allocate shared memory inside the sandbox,
for example as backing memory for ArrayBuffers.

Bug: chromium:1218005
Change-Id: I4f1f50baec50734e846496cff78046e4fffe75c5
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3383777Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79123}