This will ensure that the Isolate tear-down sequence runs the
destructors even if the first callback already ran. The second callback
might never be executed, since it runs on a background thread which
might not be executed before tear-down of the Isolate.

R=titzer@chromium.org, mlippautz@chromium.org

Bug: v8:8208, chromium:893549
Change-Id: Ibc148e71bc06936033c5b0f0e70bbb2b5a8e8094
Reviewed-on: https://chromium-review.googlesource.com/c/1276629Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56592}