Ensure that scheduled tasks are executed during db thread shutdown

Since closing the databases on the database thread can schedule
new cleanup tasks, we need to postTask the notificatino that we're done.

BUG=333058
R=michaeln@chromium.org, abarth@chromium.org

Review URL: https://codereview.chromium.org/183093002

git-svn-id: svn://svn.chromium.org/blink/trunk@168059 bbb929c8-8fbe-4397-9dbb-9b2b20218538