Added UTFGridJSON to the TileJSON externs

externs/tilejson.js

@@ -1,6 +1,7 @@
 /**
  * @externs
  * @see https://github.com/mapbox/tilejson-spec
+ * @see https://github.com/mapbox/utfgrid-spec
  */
 
 
@@ -93,3 +94,28 @@ TileJSON.prototype.bounds;
  * @type {!Array.<number>|undefined}
  */
 TileJSON.prototype.center;
+
+
+
+/**
+ * @constructor
+ */
+var UTFGridJSON = function() {};
+
+
+/**
+ * @type {!Array.<string>}
+ */
+UTFGridJSON.prototype.grid;
+
+
+/**
+ * @type {!Array.<string>}
+ */
+UTFGridJSON.prototype.keys;
+
+
+/**
+ * @type {!Object.<string, Object>|undefined}
+ */
+UTFGridJSON.prototype.data;