Fix cluster-fuzz regression when getting message from Worker

Fix cluster-fuzz regression when getting message from Worker

The issue is that Worker.prototype.terminate was deleting the C++ Worker
object, and then Worker.prototype.getMessage was trying to read messages from
the queue.

The simplest solution is to keep workers in a zombie state when they have been
terminated. They won't be reaped until Shell::CleanupWorkers is called.

I've also fixed some threading issues with Workers:

* Workers can be created by another Worker, so the Shell::workers_ variable
must be protected by a mutex.

* An individual Worker can typically only be accessed by the isolate that
created it, but the main thread can always terminate it, so the Worker::state_
must be accessed in a thread-safe way.

BUG=chromium:504136
R=jochen@chromium.org
LOG=n

Committed: https://crrev.com/28b0129b03b50d0b29fc3c45452aced2f8bcbaf6
Cr-Commit-Position: refs/heads/master@{#29306}

[jochen (gone - plz use gerrit), 2015-06-25 09:35:30]

lgtm

[binji, 2015-06-25 17:58:36]

The CQ bit was checked by binji@chromium.org

[commit-bot: I haz the power, 2015-06-25 17:58:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1208733002/20001

[commit-bot: I haz the power, 2015-06-25 18:01:15]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-25 18:01:34]

Patchset 2 (id:??) landed as
https://crrev.com/28b0129b03b50d0b29fc3c45452aced2f8bcbaf6
Cr-Commit-Position: refs/heads/master@{#29306}