[cssom][css-transforms] Allow other specs to specify 'resolved value' (w3c#882)

Fixes w3c#392.

Author: zcorpan

css-transforms/Overview.bs

@@ -825,6 +825,7 @@ If two or more values are defined and either no value is a keyword, or the only
     Computes to ''50%'' (''left 50%'') for the horizontal position if the horizontal position is not otherwise specified, or ''50%'' (''top 50%'') for the vertical position if it is.
 </dl>
 
+'transform-origin' is a <a>resolved value special case property</a>.
 The <a href="https://www.w3.org/TR/cssom/#resolved-value">resolved value</a> of 'transform-origin' is the <a href="https://www.w3.org/TR/CSS21/cascade.html#used-value">used value</a> (i.e., percentages are resolved to absolute lengths).
 
 
@@ -1087,6 +1088,7 @@ The values for 'perspective-origin' represent an offset of the perspective origi
     it is.
 </dl>
 
+'perspective-origin' is a <a>resolved value special case property</a>.
 The <a>resolved value</a> of 'perspective-origin' is the <a>used value</a> (i.e., percentages are resolved to absolute lengths).
 
 <!--

cssom/Overview.bs

@@ -2741,6 +2741,9 @@ as follows:
  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>A <dfn export>resolved value special case property</dfn> defined in another specification
+ <dd>As defined in the relevant specification.
+
  <dt>Any other property
  <dd>The <a>resolved value</a> is the <a>computed value</a>.
 </dl>