auth: Refactor how authentication methods are passed to server/auth library.

milo/appengine/common/middleware.go

Index: milo/appengine/common/middleware.go
diff --git a/milo/appengine/common/middleware.go b/milo/appengine/common/middleware.go
index daef78d87690eb9cab334f9cff9f673c2a3beeb2..6b1d48875ac27fe2d527fbddf27d510d50957f2e 100644
--- a/milo/appengine/common/middleware.go
+++ b/milo/appengine/common/middleware.go
@@ -56,12 +56,13 @@ func GetTemplateBundle() *templates.Bundle {
 
 // Base returns the basic LUCI appengine middlewares.
 func Base() router.MiddlewareChain {
-	methods := auth.Authenticator{
-		&server.OAuth2Method{Scopes: []string{server.EmailScope}},
-		server.CookieAuth,
-		&server.InboundAppIDAuthMethod{},
-	}
-	m := gaemiddleware.BaseProd().Extend(auth.Use(methods), auth.Authenticate)
+	m := gaemiddleware.BaseProd().Extend(auth.Authenticate(&auth.Authenticator{
+		Methods: []auth.Method{
+			&server.OAuth2Method{Scopes: []string{server.EmailScope}},
+			server.CookieAuth,
+			&server.InboundAppIDAuthMethod{},
+		},
+	}))
 	m = m.Extend(withRequestMiddleware)
 	m = m.Extend(templates.WithTemplates(GetTemplateBundle()))
 	return m