Sync the DataTransfer* interfaces with the spec

Source/core/clipboard/DataTransferItemList.idl

Index: Source/core/clipboard/DataTransferItemList.idl
diff --git a/Source/core/clipboard/DataTransferItemList.idl b/Source/core/clipboard/DataTransferItemList.idl
index 7bdf22c9cf8216ab1c16daefa4460b1af06ef5da..c350fa08984e300ed117959e5c3de096a802e676 100644
--- a/Source/core/clipboard/DataTransferItemList.idl
+++ b/Source/core/clipboard/DataTransferItemList.idl
@@ -28,17 +28,17 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-// http://www.whatwg.org/specs/web-apps/current-work/multipage/dnd.html#datatransferitemlist
+// https://html.spec.whatwg.org/#the-datatransferitemlist-interface
 
 [
     TypeChecking=Interface,
     WillBeGarbageCollected,
 ] interface DataTransferItemList {
-    readonly attribute long length;
+    readonly attribute unsigned long length;
     [ImplementedAs=item] getter DataTransferItem (unsigned long index);
-
-    [RaisesException] DataTransferItem add(DOMString data, DOMString type);
-    DataTransferItem add(File? file);
+    [RaisesException] DataTransferItem? add(DOMString data, DOMString type);
+    // TODO(philipj): The file argument should not be nullable.
+    DataTransferItem? add(File? file);
     [RaisesException, ImplementedAs=deleteItem] void remove(unsigned long index);
     void clear();
 };