Destroy the weak script handle from a custom finalizer, to ensure that
this is done from the main thread.

R=clemensb@chromium.org

Bug: v8:10417
Change-Id: I76a27c4a6db5ef4d189febcf9d0f81f9fd6c220d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2151347Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67267}