Oilpan: Prepare to move AbstractSQLTransaction and SQLTransaction to Oilpan heap.

BUG=347902

Review URL: https://codereview.chromium.org/196533003

git-svn-id: svn://svn.chromium.org/blink/trunk@169308 bbb929c8-8fbe-4397-9dbb-9b2b20218538