Revert of d8 workers: make sure Shell::Quit is only called once (patchset #5 id:80001...

Revert of d8 workers: make sure Shell::Quit is only called once (patchset #5 id:80001 of https://codereview.chromium.org/1230403003/)

Reason for revert:
Breaks webkit_unit_tests. See http://build.chromium.org/p/client.v8.fyi/builders/V8-Blink%20Win/builds/853/steps/webkit_unit_tests/logs/stdio

Original issue's description:
> 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
>
> Committed: https://crrev.com/76184292b392d107609f21662a949b58bb1e258c
> Cr-Commit-Position: refs/heads/master@{#29654}

TBR=jarin@chromium.org,machenbach@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4279

Review URL: https://codereview.chromium.org/1235083004

Cr-Commit-Position: refs/heads/master@{#29656}