To indicate that the Isolate* in getters might not be a "real" isolate,
but rather a calculated one from GetIsolateForPtrCompr only used for
calculating the isolate root, make that function return a const Isolate*
and change field getters, Object::IsFoo predicates, and related
functions to all take a const Isolate* instead of an Isolate*

With this change, we can slightly more confidently use Objects that are
in OffThreadSpace, without having to worry too much about having an
Isolate* floating around that could accidentally be used.

This is a slight abuse of const semantics, but it allows implicit
conversion from Isolate* arguments to the const Isolate* parameter.

Bug: v8:7703
Bug: chromium:1011762
Change-Id: I54d4a65d2299477195f4d754cabe64ce34fdaa4c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1939455
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65199}