Make the deserializer class templated on Isolate/LocalIsolate. This
allows the ObjectSerializer to be split into a main-thread and offthread
variant, with the latter taking a LocalIsolate.

Eventually, we probably want to anyway split off the code-cache de/serializer
to a separate implementation (for various reasons), and this the only one that
wants off-thread finalization, and at this point the deserializer can revert
back to being un-templated, used only for bootstrapping. However, this is the
simplest way, for now, to enable off-thread deserialization.

Bug: chromium:1075999
Change-Id: I49c0d2c5409f0aa58183673785296756c3714f22
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2562254Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75834}