The flag lets d8 wait for async compilation of WebAssembly to finish
before d8 shuts down.

R=clemensh@chromium.org
CC=​kimanh@google.com

Change-Id: I4cf59a1f35cc5a0ecaf769d2745df5e2e14eb509
Reviewed-on: https://chromium-review.googlesource.com/966031
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51999}