Terminate and wait the worker thread in ~WebEmbeddedWorkerImpl()

It is possible that termination of worker thread is not triggered by the
time of WebEmbeddedWorkerImpl destruction. We should call
WorkerThread::stop() before waiting for the termination of worker thread.

BUG=426332

Review URL: https://codereview.chromium.org/704063002

git-svn-id: svn://svn.chromium.org/blink/trunk@184900 bbb929c8-8fbe-4397-9dbb-9b2b20218538