Merge pull request #4506 from Leaflet/layer-id-public

Switch to public api for getting the layer id in debug

debug/map/popup.html

@@ -53,7 +53,7 @@
     ]);
 
     features.bindPopup(function(layer){
-      return 'Leaflet ID is ' + layer._leaflet_id;
+      return 'Leaflet ID is ' + features.getLayerId(layer);
     }).addTo(map);
 
     var content = L.DomUtil.create('p', 'custom-popup');