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

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}

[binji, 2015-07-14 18:09:34]

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

[commit-bot: I haz the power, 2015-07-14 18:09:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1235083004/1

[commit-bot: I haz the power, 2015-07-14 18:09:52]

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

[commit-bot: I haz the power, 2015-07-14 18:09:53]

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.

[adamk, 2015-07-14 18:13:22]

The CQ bit was checked by adamk@chromium.org

[adamk, 2015-07-14 18:13:27]

lgtm

[commit-bot: I haz the power, 2015-07-14 18:13:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1235083004/1

[commit-bot: I haz the power, 2015-07-14 18:13:55]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-14 18:14:11]

Patchset 1 (id:??) landed as
https://crrev.com/6b2c6eb75678747afca59b4a78ace597e218145d
Cr-Commit-Position: refs/heads/master@{#29656}

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

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1224203004/ by binji@chromium.org.

The reason for reverting is: 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/....