Get rid of PageGroupIndexedDatabase as there is only one page group in Blink

We just create the indexedDB factory directly in DOMWindowIndexedDatabase
now. This is part of the work on getting rid of PageGroup.

BUG=288249

Review URL: https://codereview.chromium.org/168603003

git-svn-id: svn://svn.chromium.org/blink/trunk@167521 bbb929c8-8fbe-4397-9dbb-9b2b20218538