用Iframe集体加载图片

```js
var ifm = document.createElement('iframe')
    document.body.appendChild(ifm)
    var img = document.createElement('img')
    img.src = 'https://web.500px.com/static/media/editors_choice_3@2x.e96bc972.png'
    ifm.contentWindow.document.body.appendChild(img)
    setTimeout(function () {
      console.log('stop')
      ifm.contentWindow.stop()
    }, 100)
```