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}