d8 workers: make sure Shell::Quit is only called once

d8 workers: make sure Shell::Quit is only called once

When running with isolates, Quit can be called simultaneously by two threads.
If this happens, then both threads try to clean up the Workers, which could
crash.

BUG=v8:4279
R=jarin@chromium.org
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1230403003

Cr-Commit-Position: refs/heads/master@{#29654}