[cssom] Make getComputedStyle return rules of the node's document.

Fixes w3c#2403

Author: emilio

cssom-1/Overview.bs

@@ -2767,10 +2767,7 @@ partial interface Window {
 The <dfn method for=Window>getComputedStyle(<var>elt</var>, <var>pseudoElt</var>)</dfn> method must run these
 steps:
 <ol>
- <li>Let <var>doc</var> be the
- {{Document}}
- associated with the <code>Window</code> object on
- which the method was invoked.
+ <li>Let <var>doc</var> be <var>elt</var>'s <a>node document</a>.
  <li>Let <var>obj</var> be <var>elt</var>.
  <li>If <var>pseudoElt</var> is as an
  <a>ASCII case-insensitive</a> match for
@@ -2790,11 +2787,6 @@ steps:
    <dt><a for="CSSStyleDeclaration">declarations</a>
    <dd>All longhand properties that are <a lt="supported CSS property">supported CSS properties</a>, in lexicographical order, with the value being
    the <a>resolved value</a> computed for <var>obj</var> using the style rules associated with <var>doc</var>.
-
-   Note: This means that even if <var>obj</var> is in a different document (e.g. one fetched via <code>XMLHttpRequest</code>) it will still use
-   the style rules associated with the document that is associated with the global object on which
-   {{Window/getComputedStyle()}} was invoked to compute the <a>CSS declaration block</a>.
-
    <dt><a for="CSSStyleDeclaration">parent CSS rule</a>
    <dd>Null.
    <dt><a for="CSSStyleDeclaration">owner node</a>