Add an explicit FreeUnreferencedObject() call that can be used to
reclaim objects that are guaranteed to not be referenced anymore
by the embedder. It is up to the embedder to ensure correctness.

Change-Id: I7f2d86d9639e8b805f79a8fd0a346903f63171e5
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2737301
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73262}