This adds new methods AllocateSharedPages, ReserveForSharedMemoryMapping
and CanAllocateSharedPages to v8::PageAllocator, which if overridden
allows the platform to declare that it supports allocation and remapping
of shared memory.

This interface is currently a work in progress so the new methods are
marked "INTERNAL ONLY" and they may change without being first
deprecated.

An implementation of PageAllocator is provided that can allocate and map
shared memory on Linux and Android, but no other platforms are yet
supported. While Windows is not supported the interface has been
designed to make this possible as AllocateSharedPages returns a
SharedMemory object that wraps the shared memory and provides its own
remap function. This should allow the SharedMemory object on windows to
contain a mapping a to hFileMappingObject as required by
MapViewOfFileEx.

Bug: v8:10454
Change-Id: I2e601d49ea14da44867a102c823fa4e341cf0dab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2306789Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69154}