Shorthand properties of computed CSSStyleDeclaration don't return correct values

```html
<div id="test" style="overflow: hidden"></div>
<script>onload = function() {
  var div = document.getElementById('test');
  console.log(getComputedStyle(div).overflow)
  console.log(getComputedStyle(div).overflowX);
};
</script>
```
In Firefox, this shows `hidden` twice. In Servo this shows an empty string, followed by `hidden`.