Fix a race condition during worker thread initialization

Fix a race condition during worker thread initialization

This patch fixes a race condition which can result in a worker thread
accessing freed memory during its initialization. One possible sequence
of events is:

1. The main thread creates the worker thread and posts a task to run
   WorkerThread::initialize().

2. Immediately after this, the main thread calls
   WorkerThread::terminateAndWait() to shut down the worker thread.

3. WorkerThread::terminateAndWait() notices that the worker hasn't
   initialized yet and signals m_terminationEvent.

4. The caller of terminateAndWait() assumes that the WorkerThread has
   been terminated and deallocates either the WorkerThread itself or
   some object that it depends on.

5. Finally, WorkerThread::initialize starts to run on the worker thread,
   accessing memory freed in step #4.

The fix is to always signal m_terminationEvent from the worker thread to
guarantee that we don't think the thread terminated before it actually
did.

BUG=499153

Review URL: https://codereview.chromium.org/1184833002

git-svn-id: svn://svn.chromium.org/blink/trunk@197196 bbb929c8-8fbe-4397-9dbb-9b2b20218538