We can actually prepare everything in the background, all that remains
to do in the finisher task (on the main thread) is actually adding the
code to the {NativeModule}.
As a next step, even that should happen in the background.

R=mstarzinger@chromium.org

Bug: v8:6600
Change-Id: I570f99a9aa7dc7e324046da36cca9b4297f1bc5e
Reviewed-on: https://chromium-review.googlesource.com/1126391
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54239}