This CL introduces an ExplicitRealmScope in d8 for entering an existing
Realm on demand.

Bug: v8:11525, v8:11706
Change-Id: I3b556aed85fc615bb5efbd4a072e075534617258
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2936602
Commit-Queue: Vicky Kontoura <vkont@google.com>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74946}