Adds support for main-thread handling of JSMember during the
atomic pause.

Follow-ups for later:
- Copy/Move/Heterogenous assignment
- Write barrier
- Atomic handling for concurrent processing.

Bug: chromium:1056170
Change-Id: Ia9ac4599ca85cf7cc2d67066e89485744d7d56b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2289781
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68813}