MaybeObjectHandle already has an API for making a weak pointer.

BUG=v8:7308

Change-Id: I9f8390e0dffec850a24391e7f50a455b86b217f5
Reviewed-on: https://chromium-review.googlesource.com/1125922Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54247}