Introduce an OffThreadTransferHandle (and OffThreadTransferMaybeHandle),
which points to a piece of storage known to (and owned by) the
OffThreadIsolate. On Publish, the OffThreadIsolate converts this storage
stub from a raw pointer to an off-thread object into a main-thread
Handle.

This allows users of an OffThreadIsolate to not have to worry (as much)
about the dance of saving raw object pointers before off-thread finishes
and converting those to Handles before it off-thread isolate is
published.

Bug: chromium:1011762
Change-Id: I7ceb4ed85e770bd6e1867649188597bbcaedb32d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2161066
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67337}