How to zoom in/out the content of a WebView programmatically?

I am trying to programmatically zoom in/out the content of a WebView object in my app. But I can't find a nice way to do this. Can anyone give me some help? It's better if there can be some开发者_如何学Go code samples:)

Finally, lollypop supports zoomby(), but under API 21 zoomin/zoomout are only ways.