[cssom] Fix getComputedStyle with over-constrained box-offset properties

When left/right or top/bottom are over-constrained, getComputedStyle
needs to return the computed value rather than the used value.

Fixes w3c#203.

Author: zcorpan

cssom/Overview.bs

@@ -1460,7 +1460,7 @@ To <dfn>serialize a CSS rule</dfn>, perform one of the following in accordance w
 
  <dt>{{CSSNamespaceRule}}
  <dd>The literal string "<code>@namespace</code>", followed by a single SPACE
- (U+0020), followed by the 
+ (U+0020), followed by the
  <a lt="serialize an identifier">serialization as an identifier</a> of the
  {{CSSNamespaceRule/prefix}} attribute (if
  any), followed by a single SPACE (U+0020) if there is a prefix, followed by the
@@ -2689,11 +2689,9 @@ as follows:
  <dt>'left'
  <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'', the <a>resolved value</a> is the
- <a>used value</a>. Otherwise the <a>resolved value</a> is the
- computed value.
+ <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, the <a>resolved
+ value</a> is the <a>used value</a>. Otherwise the <a>resolved value</a> is the computed value.
 
  <dt>Any other property
  <dd>The <a>resolved value</a> is the <a>computed value</a>.
@@ -2810,6 +2808,7 @@ Ian Hickson,
 John Daggett,
 Lachlan Hunt,
 Mike Sherov,
+Myles C. Maxfield,
 Morten Stenshorne,
 Ms2ger,
 Øyvind Stenhaug,