IndexedDB: Make webkitGetDatabaseNames source null

The standard IDBFactory.open() and IDBFactory.deleteDatabase() methods
set the source of the returned request to |null|. The non-standard
IDBFactory.webkitGetDatabaseNames() method was setting the source to
the factory, which adds special cases to IDBAny for no benefit. Make
the source |null| like the other methods, and tidy up IDBAny.

R=dgrogan@chromium.org
TBR=haraken@chromium.org

Review URL: https://codereview.chromium.org/102543004

git-svn-id: svn://svn.chromium.org/blink/trunk@163189 bbb929c8-8fbe-4397-9dbb-9b2b20218538