Add tileLoadFunction option to ol.source.XYZ

src/objectliterals.jsdoc

@@ -616,6 +616,8 @@
  * @property {number|undefined} maxZoom Optional max zoom level.  The default is
  *     18.
  * @property {number|undefined} minZoom Unsupported (TODO: remove this).
+ * @property {ol.TileLoadFunctionType|undefined} tileLoadFunction Optional
+ *     function to load a tile given a URL.
  * @property {ol.TileUrlFunctionType|undefined} tileUrlFunction Optional
  *     function to get tile URL given a tile coordinate and the projection.
  *     Required if url or urls are not provided.

src/ol/source/xyzsource.js

@@ -31,6 +31,7 @@ ol.source.XYZ = function(options) {
     logo: options.logo,
     projection: projection,
     tileGrid: tileGrid,
+    tileLoadFunction: options.tileLoadFunction,
     tileUrlFunction: ol.TileUrlFunction.nullTileUrlFunction
   });