Oilpan: Replace remaining RefPtr<IDBTransaction> with Oilpan types.

BUG=366528

Review URL: https://codereview.chromium.org/269883002

git-svn-id: svn://svn.chromium.org/blink/trunk@173281 bbb929c8-8fbe-4397-9dbb-9b2b20218538