Oilpan: Add a comment to a Persistent data member in DatabaseSync.

BUG=347902
NOTRY=true

Review URL: https://codereview.chromium.org/203303009

git-svn-id: svn://svn.chromium.org/blink/trunk@169527 bbb929c8-8fbe-4397-9dbb-9b2b20218538