When quit() is called, d8 shell exits without cleanup. If a worker is running,
it might be holding the context_mutex_, which if destroyed will DCHECK.
BUG=4279
R=jarin@chromium.org
LOG=n
Review URL: https://codereview.chromium.org/1231473002
Cr-Commit-Position: refs/heads/master@{#29557}
