why heatmaplayer is a img not canvas in chrome

[ldllidonglin]

in the demo : http://esri.github.io/heatmap-layer-js/

The result is not rander in canvas element,but rander be a img element
such as

<img id="map_heatLayer_1463388826775" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgA.. />

my browser is chrome 49.0.2623.87,it's support canvas.