In password_protection_service.cc GetHostNameWithHTTPScheme() uses url.HostNoBrackets() to get the hostname. So for a URL containing an IPv6 address like https://[::1]/foo it will retrieve a hostname of "::1", then concatenate it with http:// to produce http://::1 which is an invalid URL.

I couldn't see any tests for IPv6 literal addresses so I can't tell whether or not this has an impact on the correctness of the function. It seems unlikely that creating invalid URLs is the intent of the function, but if it is it would be helpful to readers if it was mentioned in a comment.

It should probably be using url.host() rather than HostNoBrackets(). Or host_piece() for less string copies.

The comment on the function says
// Given a URL of either http or https scheme, return its scheme://hostname.
// e.g., "https://www.foo.com:80/bar/test.cgi" -> "http://www.foo.com".

I think instead of "its scheme://hostname" this should say "http://hostname".