ServiceWorker: Fix data race in shutdown sequence

To avoid Worker thread's termination code touching released objects
owned by WebEmbeddedWorkerImpl, join the worker thread in the destructor
of WebEmbeddedWorkerImpl.

BUG=426332

Review URL: https://codereview.chromium.org/684223004

git-svn-id: svn://svn.chromium.org/blink/trunk@184647 bbb929c8-8fbe-4397-9dbb-9b2b20218538