Instead of spawning several foreground tasks, execute the work in a
single chunk.
This will allow us in a follow-up step to remove the deferred handle
scope and pass the handles directly.

R=ahaas@chromium.org

Bug: v8:7921, v8:8423
Change-Id: I4dae6241e2a33e89e304a04cb67c2229c34f7b99
Reviewed-on: https://chromium-review.googlesource.com/c/1402545
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58677}