Expose DataTransferItemList type to JavaScript

Expose DataTransferItemList type to JavaScript by dropping the [NoInterfaceObject]
IDL extended attribute on this interface, as per 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.

BUG=271424

Review URL: https://chromiumcodereview.appspot.com/22854002

git-svn-id: svn://svn.chromium.org/blink/trunk@156022 bbb929c8-8fbe-4397-9dbb-9b2b20218538