[cssom] Use CSS/Parse in getComputedStyle().

Fixes w3c#3875

Author: emilio

cssom-1/Overview.bs

@@ -2807,10 +2807,9 @@ steps:
  <li>Let <var>obj</var> be <var>elt</var>.
  <li>If <var>pseudoElt</var> is provided, and it's not the empty string, then:
   <ol>
-    <li>If it is an <a>ASCII case-insensitive</a> match for a pseudo-element
-    that the UA supports, other than ''::slotted()'', let <var>obj</var>
-    be the given pseudo-element of <var>elt</var>.
-    Otherwise <a>throw</a> a {{TypeError}} exception.
+    <li>[=CSS/Parse=] <var>pseudoElt</var> as a <<pseudo-element-selector>>, and let <var>type</var> be the result.
+    <li>If <var>type</var> is failure, or is an ''::slotted()'' or ''::part()'' pseudo-element, <a>throw</a> a {{TypeError}} exception.
+    <li>Otherwise let <var>obj</var> be the given pseudo-element of <var>elt</var>.
   </ol>
 
   Note: CSS2 pseudo-elements should match both the double and single-colon