Reimplement downloadItem.getFilename API.

Previously, the suggested file name(Always 'empty') returned by
'download_item->GetSuggestedFilename' is not the same with the default one saved
in local disk.

The patch reimplement this API allowing it to return the default file name, which
is more expected from user.

spec/static/main.js

@@ -89,7 +89,8 @@ app.on('ready', function() {
                                     item.getMimeType(),
                                     item.getReceivedBytes(),
                                     item.getTotalBytes(),
-                                    item.getContentDisposition());
+                                    item.getContentDisposition(),
+                                    item.getFilename());
           });
           if (need_cancel)
             item.cancel();