Extract getTile middleware from controller's context

lib/cartodb/controllers/named_maps.js

@@ -43,6 +43,7 @@ module.exports = NamedMapsController;
 NamedMapsController.prototype.register = function(app) {
     const tileOptions = {
         namedMapProviderCache: this.namedMapProviderCache,
+        tileBackend: this.tileBackend,
         label: 'NAMED_MAP_TILE'
     };
 
@@ -53,7 +54,7 @@ NamedMapsController.prototype.register = function(app) {
         this.prepareContext,
         getNamedMapProvider(tileOptions),
         getAffectedTables(),
-        this.getTile(tileOptions),
+        getTile(tileOptions),
         this.setSurrogateKey(),
         this.setCacheChannelHeader(),
         this.setLastModifiedHeader(),
@@ -194,11 +195,11 @@ function prepareLayerFilterFromPreviewLayers ({ namedMapProviderCache, label })
     };
 };
 
-NamedMapsController.prototype.getTile = function ({ label }) {
+function getTile ({ tileBackend, label }) {
     return function getTileMiddleware (req, res, next) {
         const { namedMapProvider } = res.locals;
 
-        this.tileBackend.getTile(namedMapProvider, req.params, (err, tile, headers, stats) => {
+        tileBackend.getTile(namedMapProvider, req.params, (err, tile, headers, stats) => {
             req.profiler.add(stats);
 
             if (err) {
@@ -212,7 +213,7 @@ NamedMapsController.prototype.getTile = function ({ label }) {
 
             next();
         });
-    }.bind(this);
+    };
 };
 
 NamedMapsController.prototype.getStaticImageOptions = function () {