v8: backport 4f82f1d948c from upstream v8

Original commit message:
    [Api] Add an idle time garbage collection callback flag to GCCallbackFlags.

    BUG=chromium:718484

    Review-Url: https://codereview.chromium.org/2867073002
    Cr-Commit-Position: refs/heads/master@{#45167}

PR-URL: https://github.com/nodejs/node/pull/13217


Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>