Avoid leaking because the persistent handle isn't released. To further
clarify ownership, the v8 side owns now completely the promise.

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ief9e44e60235fe6199fc4884ad1ccbd9e34cce8a
Reviewed-on: https://chromium-review.googlesource.com/591067Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46978}