Make the WebThread be the last member of WorkerThread

Currently, the EmbeddedWorkerDispatcher forcibly destroys all workers it
owns without properly shutting the thread down. Therefore it's important
that we don't start to destruct other members of WorkerThread before the
WebThread has joined.

BUG=400511
R=horo@chromium.org,haraken@chromium.org,nasko@chromium.org

Review URL: https://codereview.chromium.org/451563003

git-svn-id: svn://svn.chromium.org/blink/trunk@179699 bbb929c8-8fbe-4397-9dbb-9b2b20218538