IndexedDB: Move DataLoss enum off of WebIDBCallbacks

The WebIDBCallbacks interface has grown an implementation, so it is no
longer appropriate to reference it from chromium's content/browser.
Give the enum a new home, with temporary support until chromium is
updated.

BUG=234277
R=jamesr@chromium.org,alecflett@chromium.org,dgrogan@chromium.org

Review URL: https://codereview.chromium.org/63163005

git-svn-id: svn://svn.chromium.org/blink/trunk@161842 bbb929c8-8fbe-4397-9dbb-9b2b20218538