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

Review URL: https://codereview.chromium.org/1208733002

Cr-Commit-Position: refs/heads/master@{#29306}