[ol.layer.TileLayer] add a getTileSize function

src/ol/layer/TileLayer.js

@@ -108,6 +108,14 @@ ol.layer.TileLayer.prototype.getExtent = function() {
     return null;
 };
 
+/**
+ * Get tile size.
+ * @return {Array.<number>}
+ */
+ol.layer.TileLayer.prototype.getTileSize = function() {
+    return [this.tileWidth_, this.tileHeight_];
+};
+
 /**
  * Get tile origin.
  * @return {Array.<number>}