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 refered to is dead.

Bug: chromium:924220
Change-Id: I2ab7c3afcbe22988791faef406c284db03a43caf
Reviewed-on: https://chromium-review.googlesource.com/c/1430101
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59040}