Adds a cross-thread reference for strongly and weakly retaining
objects on a thread other than the thread that owns the object.

The intended use of the reference is by setting it up on the
originating thread, holding the object alive from another thread, and
ultimately accessing the object again on the originating thread.

The reference has known caveats:
- It's unsafe to use when the heap may terminate;
- It's unsafe to transitively reach through the graph because of
  compaction;

Change-Id: I84fbdde69a099eb54af5b93c34e2169915b17e64
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2436449
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70428}