Make HTTP middleware easier to use

appengine/cmd/milo/settings/settings.go

Index: appengine/cmd/milo/settings/settings.go
diff --git a/appengine/cmd/milo/settings/settings.go b/appengine/cmd/milo/settings/settings.go
index d7fb7399be6c4f605603306a8383b8dbb479a508..fe04460cc61da16d4be492805136f807c8a37df2 100644
--- a/appengine/cmd/milo/settings/settings.go
+++ b/appengine/cmd/milo/settings/settings.go
@@ -3,27 +3,27 @@
 // that can be found in the LICENSE file.
 
 package settings
 
 import (
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
 	"net/http"
 
-	"github.com/julienschmidt/httprouter"
 	"github.com/luci/gae/service/datastore"
 	"github.com/luci/luci-go/appengine/cmd/milo/model"
 	"github.com/luci/luci-go/appengine/cmd/milo/resp"
 	"github.com/luci/luci-go/server/auth"
 	"github.com/luci/luci-go/server/auth/identity"
 	"github.com/luci/luci-go/server/auth/xsrf"
+	"github.com/luci/luci-go/server/router"
 	"golang.org/x/net/context"
 )
 
 type updateReq struct {
 	Theme string
 }
 
 // GetTheme returns the chosen theme based on the current user.
 func GetTheme(c context.Context, r *http.Request) Theme {
 	cfg := getUserSettings(c)
@@ -68,21 +68,23 @@ func getCookieSettings(c context.Context, r *http.Request) *model.UserConfig {
 	}
 	err = json.Unmarshal([]byte(s), &config)
 	if err != nil {
 		panic(err)
 	}
 	return &config
 }
 
 // ChangeSettings is invoked in a POST request to settings and changes either
 // the user settings in the datastore, or the cookies if user is anon.
-func ChangeSettings(c context.Context, h http.ResponseWriter, r *http.Request, p httprouter.Params) {
+func ChangeSettings(ctx *router.Context) {
+	c, h, r := ctx.Context, ctx.Writer, ctx.Request
+
 	// First, check XSRF token.
 	err := xsrf.Check(c, r.FormValue("xsrf_token"))
 	if err != nil {
 		h.WriteHeader(http.StatusUnauthorized)
 		h.Write([]byte("Failed XSRF check."))
 		return
 	}
 
 	u := &updateReq{
 		Theme: r.FormValue("theme"),