DevTools: WebSQL: show rowid column.

This predefined column will be essential to add "hide columns" feature.

BUG=272582

Review URL: https://codereview.chromium.org/648333004

git-svn-id: svn://svn.chromium.org/blink/trunk@184436 bbb929c8-8fbe-4397-9dbb-9b2b20218538