Fix 'grids' type declaration.

grids property is optional.
see: https://github.com/mapbox/tilejson-spec/blob/master/2.0.0/README.md

externs/tilejson.js

@@ -66,7 +66,7 @@ TileJSON.prototype.tiles;
 
 
 /**
- * @type {!Array.<string>}
+ * @type {!Array.<string>|undefined}
  */
 TileJSON.prototype.grids;