[cssom] Fix getComputedStyle for display:contents

display:contents should act like display:none in getComputedStyle.

Fixes #482.

Author: zcorpan

cssom/Overview.bs

@@ -2699,8 +2699,8 @@ as follows:
  <dt>'padding'
  <dt>'width'
  <dd>If the property applies to the element or pseudo-element and the <a>resolved value</a> of the
- 'display' property is not ''display/none'', then the <a>resolved value</a> is the <a>used
- value</a>. Otherwise the <a>resolved value</a> is the <a>computed value</a>.
+ 'display' property is not ''display/none'' or ''display/contents'', then the <a>resolved value</a>
+ is the <a>used value</a>. Otherwise the <a>resolved value</a> is the <a>computed value</a>.
 
  <dt>'bottom'
  <dt>'left'
@@ -2711,9 +2711,9 @@ as follows:
  <dt>'right'
  <dt>'top'
  <dd>If the property applies to a positioned element and the <a>resolved value</a> of the 'display'
- property is not ''display/none'', and the property is not over-constrained, then the <a>resolved
- value</a> is the <a>used value</a>. Otherwise the <a>resolved value</a> is the <a>computed
- value</a>.
+ property is not ''display/none'' or ''display/contents'', and the property is not over-constrained,
+ then the <a>resolved value</a> is the <a>used value</a>. Otherwise the <a>resolved value</a> is the
+ <a>computed value</a>.
 
  <dt>Any other property
  <dd>The <a>resolved value</a> is the <a>computed value</a>.