Have DataTransferItemList.add() methods return the added item

Have DataTransferItemList.add() methods return the added item to match
the latest specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/dnd.html#datatransferitemlist

DataTransferItemList is not implemented in Firefox 23 or IE10 yet.

BUG=271424

Review URL: https://chromiumcodereview.appspot.com/22837006

git-svn-id: svn://svn.chromium.org/blink/trunk@156094 bbb929c8-8fbe-4397-9dbb-9b2b20218538