www.youtube .com says: Your computer is infected. You have to check it with antivirus.
FIX: Remove Infinity New Tab, in no time!
Related info: 1. https://www.reddit.com/r/chrome/comments/6dluxp/your_computer_is_infected_you_have_to_check_it/
Are you using Infinity New Tab as an extension? there's another post on this sub mentioning that some malicious code was put into the last update of the extension, which may be causing this pop-up. Infinity New Tab was also removed from the Chrome Web Store. Had this problem all evening deleting the extension seemed to stop the pop-up.