... for the cases when the Isolate is necessary only for external
pointers decoding. This will avoid unnecessary calls to non-inlined
IsolateFromNeverReadOnlySpaceObject().

Bug: v8:10391
Change-Id: I0a299c8a44d5845f26cf704ce53555bf07c93f8d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2198978Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67771}