Check whether the worker was terminated before accessing its backing thread

Check whether the worker was terminated before accessing its backing thread

This patch fixes a race condition in WorkerThread initialization which
can lead to a null pointer dereference. An example triggering sequence
of events is:

Main thread                                  Worker thread
-----------                                  -------------
1. ServiceWorkerThread::start
   |                              postTask
    '----------------------------------------.
                                             |
                                             V

2. ~ServiceWorkerThread                      3. ServiceWorkerThread::initialize
   => ~WebThreadSupportingGC                    => derefs backingThread().platformThread()
   => backingThread().platformThread()          => segfault
      becomes null.

In other words, the worker thread is terminated before it has a chance
to run the posted WorkerThread::initialize() function.

The fix is to check whether the thread was terminated before assuming
the backing thread is still valid.

BUG=492592

Review URL: https://codereview.chromium.org/1149833011

git-svn-id: svn://svn.chromium.org/blink/trunk@196058 bbb929c8-8fbe-4397-9dbb-9b2b20218538