With https://chromium-review.googlesource.com/c/532993/ the isolate
can shutdown without a crash even when an async compile job is still
active. This should allow us to turn on async compilation again.

R=mtrofin@chromium.org, bradnelson@chromium.org

Change-Id: I822112c4f2a7a32133aa1d086881ed77baf42f76
Reviewed-on: https://chromium-review.googlesource.com/533293Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46222}