It appears when a resource with invalid or truncated url is added. We attempt to parse the URL and show the domain name as a top level folder name in Sources panel navigator.
For instance this could happen for a script/stylesheet with sourceURL.
In your case you only have an (index) entry under this folder, so it looks like a resource with an empty URL was added somehow.
This could happen if you have some extensions installed. Is that your case? Could you please file a bug report on
http://crbug.com/new ?