Reland "[gin] Allow posting non-nestable tasks through V8"

This is a reland of 7f08809af15106c04feb6188b5d54c93af19472b

Original change's description:
> [gin] Allow posting non-nestable tasks through V8
>
> Will be used to allow V8 posting top-level tasks.
>
> Bug: chromium:843903
> Change-Id: I97caa705b561fa8832ef8a11abfc3eefe47ec3f8
> Reviewed-on: https://chromium-review.googlesource.com/c/1273140
> Reviewed-by: Jochen Eisinger <jochen@chromium.org>
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#598802}

Tbr: jochen@chromium.org
Bug: chromium:843903
Change-Id: I52e95076a32e0648c4c77530689b41757f7420c8
Reviewed-on: https://chromium-review.googlesource.com/c/1277798
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599543}
4 files changed