PasswordManager implements the LoginModel interface to serve http auth forms. But the implementation is largely independent of the rest of PasswordManager.

Instead, we should create a separate class implementing LoginModel, and move also PasswordManager::AutofillHttpAuth and PasswordManager::observers_ into it. This will make PasswordManager itself clearer and unittesting easier.

While there seems to be no other subclass of LoginModel than PasswordManager, we should probably keep the pulled-out implementation still a separate subclass of LoginModel (as opposed to just putting it inside LoginModel). Having the separate interface will again make unittesting easier.

It is not clear ATM whether PasswordManager should own the LoginModel implementation, or whether they should be completely independent. The latter is preferable, if possible.