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

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
Committed: https://crrev.com/4073657ec828617beaeae6f8088449ca3fd6f6f7
Cr-Commit-Position: refs/heads/master@{#29663}

[binji, 2015-07-14 19:20:22]

Created Reland of d8 workers: make sure Shell::Quit is only called once

[commit-bot: I haz the power, 2015-07-14 19:20:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1224203004/1

[commit-bot: I haz the power, 2015-07-14 19:20:44]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-14 19:20:45]

No L-G-T-M from a valid reviewer yet. Only full committers are accepted.
Even if an L-G-T-M may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Michael Achenbach, 2015-07-14 19:52:51]

lgtm

[Michael Achenbach, 2015-07-14 19:53:26]

Something on the blink side broke the unit tests.

[binji, 2015-07-14 23:03:56]

The CQ bit was checked by binji@chromium.org

[commit-bot: I haz the power, 2015-07-14 23:04:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1224203004/1

[commit-bot: I haz the power, 2015-07-14 23:04:25]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-14 23:04:44]

Patchset 1 (id:??) landed as
https://crrev.com/4073657ec828617beaeae6f8088449ca3fd6f6f7
Cr-Commit-Position: refs/heads/master@{#29663}