d8 workers: fix race on quit() with context_mutex_

d8 workers: fix race on quit() with context_mutex_

When quit() is called, d8 shell exits without cleanup. If a worker is running,
it might be holding the context_mutex_, which if destroyed will DCHECK.

BUG=v8:4279
R=jarin@chromium.org
LOG=n

Committed: https://crrev.com/d42e81d587c725e38da3a53fc581db0e3476e408
Cr-Commit-Position: refs/heads/master@{#29557}

[Jarin, 2015-07-08 06:54:35]

lgtm

[binji, 2015-07-09 18:52:08]

The CQ bit was checked by binji@chromium.org

[commit-bot: I haz the power, 2015-07-09 18:52:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1231473002/40001

[commit-bot: I haz the power, 2015-07-09 19:30:26]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-07-09 19:30:37]

Patchset 3 (id:??) landed as
https://crrev.com/d42e81d587c725e38da3a53fc581db0e3476e408
Cr-Commit-Position: refs/heads/master@{#29557}