WorkerThread::terminated() needs to acquire a mutex

This is a speculative fix for bug 418040.
WorkerThread needs to grab m_threadCreationMutex when accessing WorkerThread::m_terminated.

BUG=418040
TEST=none. I couldn't reproduce it even with TSan.

Review URL: https://codereview.chromium.org/615893002

git-svn-id: svn://svn.chromium.org/blink/trunk@182926 bbb929c8-8fbe-4397-9dbb-9b2b20218538