Reland of d8 workers: make sure Shell::Quit is only called once (patchset #1 id:1 of https://codereview.chromium.org/1235083004/)

Reason for revert:
Looks like the failure is unrelated to my change (still fails after the revert). See http://build.chromium.org/p/client.v8.fyi/builders/V8-Blink%20Win/builds/856/steps/webkit_unit_tests/logs/stdio

Original issue's description:
> 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
>
> Committed: https://crrev.com/6b2c6eb75678747afca59b4a78ace597e218145d
> Cr-Commit-Position: refs/heads/master@{#29656}

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

Review URL: https://codereview.chromium.org/1224203004

Cr-Commit-Position: refs/heads/master@{#29663}