Oilpan: Prepare to move DatabaseThread and SQLTransactionCoordinator to Oilpan heap.

BUG=347902

Review URL: https://codereview.chromium.org/201083003

git-svn-id: svn://svn.chromium.org/blink/trunk@169594 bbb929c8-8fbe-4397-9dbb-9b2b20218538