- Allow downcasting construciton and assignment;
- Add WeakCrossThreadPersistent::Lock() that safely retrieves a strong
  handle for a weak reference;

Bug: chromium:1056170
Change-Id: I5f8d85a87c9955506dd87723ffb4c80d66770c04
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2663160
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72468}