Async compilation causes problem when the isolate shuts down
concurrently.

R=hablich@chromium.org

Bug: chromium:764313
Change-Id: I35b5ba9bdb70291b53e87a5323f5408930f01ddd
Reviewed-on: https://chromium-review.googlesource.com/687076Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48188}