Add an `array_buffer_allocator_shared` field to the
`Isolate::CreateParams` struct that allows embedders to share
ownership of the ArrayBuffer::Allocator with V8, and which in
particular means that when this method is used that the
BackingStore deleter will not perform an use-after-free access to the
Allocator under certain circumstances.

For Background:

tl;dr: This is necessary for Node.js to perform the transition to
V8 7.9, because of the way that ArrayBuffer::Allocators and their
lifetimes currently work there.

In Node.js, each Worker thread has its own ArrayBuffer::Allocator.
Changing that would currently be impractical, as each allocator
depends on per-Isolate state. However, now that backing stores
are managed globally and keep a pointer to the original
ArrayBuffer::Allocator, this means that when transferring an
ArrayBuffer (e.g. from one Worker to another through postMessage()),
the original Allocator has to be kept alive until the ArrayBuffer
no longer exists in the receiving Isolate (or until that Isolate
is disposed). See [1] for an example Node.js test that fails with
V8 7.9.

This problem also existed for SharedArrayBuffers, where Node.js
was broken by V8 earlier for the same reasons (see [2] for the bug
report on that and [3] for the resolution in Node.js).
For SharedArrayBuffers, we already had extensive tracking logic,
so adding a shared_ptr to keep alive the ArrayBuffer::Allocator
was not a significant amount of work. However, the mechanism for
transferring non-shared ArrayBuffers is quite different, and
it seems both easier for us and better for V8 from an API standpoint
to keep the Allocator alive from where it is being referenced.

By sharing memory with the custom deleter function/data pair,
this comes at no memory overhead.

[1]: https://github.com/nodejs/node/pull/30044
[2]: https://github.com/nodejs/node-v8/issues/115
[3]: https://github.com/nodejs/node/pull/29637

Bug: v8:9380
Change-Id: Ibc2c4fb6341b53653cbd637bd8cb3d4ac43809c7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1874347
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64542}