v8::Global may be used as a weak reference. In the case this reference is a
simple phantom reference, we need to update the internal state to be able to
clear the right slot once the object referred to is dead.

This reverts commit 18f32ca8.

Bug: chromium:924220
Change-Id: I3caec77448b0c5fcb461c8f8b5015de2978b3931
Reviewed-on: https://chromium-review.googlesource.com/c/1430015Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59055}