DOMDataStore should be cleared when shutting down worker threads

- The essence of this CL is to call DOMWrapperWorld::dispose() when shutting down WorkerScriptController. This makes sure that DOMDataStore is cleared when shutting down the worker thread.

- This CL also fixes misused RefPtr<DOMWrapperWorld> and DOMWrapperWorld*. In general, we should use RefPtr<DOMWrapperWorld> when we store DOMWrapperWorld in heap-allocated objects and when we want to pass ownership around. Otherwise, we should use DOMWrapperWorld*.

BUG=341032

Review URL: https://codereview.chromium.org/189513012

git-svn-id: svn://svn.chromium.org/blink/trunk@168874 bbb929c8-8fbe-4397-9dbb-9b2b20218538