Signal WorkerThread termination earlier

This CL resolves a race in WorkerThread::cleanup.
workerThreadTerminated() notification may release a reference to the WorkerThread instance, that causes race between the termination signal.

BUG=405420

Review URL: https://codereview.chromium.org/667813003

git-svn-id: svn://svn.chromium.org/blink/trunk@184061 bbb929c8-8fbe-4397-9dbb-9b2b20218538