WeakMember references are used in ephemerons which uses the ordinary
LivenessBroker for determining whether an object is dead or not.

Bug: chromium:1056170
Change-Id: I7f25da22637fba24603bccb76e266357b0371525
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2649042
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72346}