IndexedDB: Move connection hookup into IDBDatabase constructor

Minor DRY refactor inspired by crrev.com/18227006

BUG=234277
R=alecflett@chromium.org

Review URL: https://codereview.chromium.org/106403005

git-svn-id: svn://svn.chromium.org/blink/trunk@163367 bbb929c8-8fbe-4397-9dbb-9b2b20218538