Add a new OffThreadObjectDeserializer, which can deserialize a snapshot
into an OffThreadIsolate.

This involves templating the Deserializer base class on Isolate, and
amending OffThreadHeap to be able to create Reservations same as the
main-thread Heap can. Various off-thread incompatible methods are
stubbed out as UNREACHABLE in OffThreadIsolate overloads.

There is currently no API entry into the off-thread deserialization, but
under --stress-background-compile it now runs the CodeDeserializer (i.e.
code cache deserialization) in a background thread.

Bug: chromium:1075999

Change-Id: I2453f51ae31df4d4b6aa94b0804a9d6d3a03781e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2172741
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67799}