Use computed value instead of used value

[zcorpan]

https://quirks.spec.whatwg.org/#the-line-height-calculation-quirk

The border-right-width, border-left-width, padding-right and padding-left properties have a used value of zero.

Per servo/servo#11853 maybe used value here should instead be computed value.

[zcorpan]

cc @nox

[zcorpan]

So a case like this?

http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4301

<style>
div { width:100px }
span { padding-left:calc(10% - 10px); line-height:20px }
</style>
<div id=test><span></span><small>test</small></div>
<script>
w(getComputedStyle(test).height)
</script>

Gecko and Chromium seem to apply the quirk for this case. (If the padding-left is changed to 1px, the div's height is 20px.)