This is a reland of e24fa913
It fixes the heap verification errors by going back to using MakeThin
instead of manually creating a filler (that then makes the verifier
think that this was array left-trimming).

Original change's description:
> [offthread] Template deserializer on Isolate
>
> 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/+/2562254
> Reviewed-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}

Bug: chromium:1075999
Change-Id: I1d81fad2550a2a9f04dd0f9d8e66422d28faf378
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3043960Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75918}