Phantom handles were processed at the same time as finalizers. This 
meant that if a finalizer kept an object alive the phantom handle
was still destroyed.

This becomes a problem in the context of Blink GCs where internal
fields are roots for Blink. Prematurely destroying a phantom handle
can lead to stale pointers.

Bug: chromium:772299
Change-Id: If02365c457be8ce48379ad357cce36baa9617cfb
Reviewed-on: https://chromium-review.googlesource.com/750625Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49086}