There is no such thing as an extrepo repository. extrepo is a method of enabling an already available repository (such as deb.torproject.org, deb.whonix.org, …) in an easier way. Instead of following third party instructions on how to enable the third party repository (add apt signing key + add apt sources.list.d snippet) it simplifies that process. No third party repository with alternative Chromium versions to being with → extrepo repository cannot help either. Similar explanation: