Oilpan: Prepare to move AbstractSQLStatementBackend and SQLStatementBackend to Oilpan heap.

They are ThreadSafeRefCounted because an object is created in a context thread,
is enqueued into a SQLTransactionBackend, the SQLTransactionBackend is moved to
a database thread, then the database thread takes a SQLStatementBackend in the
queue.
We don't need to care about it in Oilpan. SQLTransactionBackend is correctly
transferred with CrossThreadPersistent.

BUG=347902

Review URL: https://codereview.chromium.org/205823002

git-svn-id: svn://svn.chromium.org/blink/trunk@169622 bbb929c8-8fbe-4397-9dbb-9b2b20218538