Allows adding a label to strong retainers in PersistentValueMapBase and
its subclasses. These labels show up in DevTools and enable identifying
strong retainers of objects.

R: ulan@chromium.org
Change-Id: Id5e19507c40e44688c82a4caec89449b563a0e8b
Reviewed-on: https://chromium-review.googlesource.com/c/1372069Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58177}