IDBFactoryBackend -> IndexedDBClient, remove indexeddb/chromium

Tidy up some naming and layering split from the WebKit days.

Review URL: https://codereview.chromium.org/225023020

git-svn-id: svn://svn.chromium.org/blink/trunk@171425 bbb929c8-8fbe-4397-9dbb-9b2b20218538