Replace the Persistent in SQLiteDatabase with a member and trace it.

R=tkent@chromium.org
BUG=

Review URL: https://codereview.chromium.org/204543005

git-svn-id: svn://svn.chromium.org/blink/trunk@169944 bbb929c8-8fbe-4397-9dbb-9b2b20218538