Update VideoOverlay.js (#5731)

* Update VideoOverlay.js

wrong keyword uses - className should be used instead
found this when testing new leaflet on ie8 - map doesn't load because of this

* Use DomUtil.addClass instead of modifying el.className

src/layer/VideoOverlay.js

@@ -39,8 +39,8 @@ export var VideoOverlay = ImageOverlay.extend({
 		var wasElementSupplied = this._url.tagName === 'VIDEO';
 		var vid = this._image = wasElementSupplied ? this._url : DomUtil.create('video');
 
-		vid.class = vid.class || '';
-		vid.class += 'leaflet-image-layer ' + (this._zoomAnimated ? 'leaflet-zoom-animated' : '');
+		DomUtil.addClass('leaflet-image-layer');
+		if (this._zoomAnimated) { DomUtil.addClass('leaflet-zoom-animated'); }
 
 		vid.onselectstart = Util.falseFn;
 		vid.onmousemove = Util.falseFn;