Some embedders primitive can trigger execution in current JavaScript
instance or in another (e.g. MessageChannel).
With this CL external async task can be local as well.

R=dgozman@chromium.org

Bug: chromium:661705
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I82c68a021c2c25bc67a706c4bfed8c1a2b2388c5
Reviewed-on: https://chromium-review.googlesource.com/792015
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49728}