Add support for DataTransferItemList.remove() method

Add support for DataTransferItemList.remove() method as per the latest
specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/dnd.html#dom-datatransferitemlist-remove

Firefox 23 and IE10 do not support DataTransferItemList yet.

BUG=271424

Review URL: https://chromiumcodereview.appspot.com/23051003

git-svn-id: svn://svn.chromium.org/blink/trunk@156015 bbb929c8-8fbe-4397-9dbb-9b2b20218538