[cssom] Editorial: make getComputedStyle's second argument nullable. Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=27633

Author: Simon Pieters

cssom/Overview.html

@@ -2965,7 +2965,7 @@ <h3 id="extensions-to-the-window-interface"><span class="secno">7.2 </span>Exten
 
 <pre class="idl">partial interface <a class="external" data-anolis-spec="html" href="https://html.spec.whatwg.org/multipage/browsers.html#window">Window</a> {
   [NewObject] <a href="#cssstyledeclaration">CSSStyleDeclaration</a> <!--
-  --><a href="#dom-window-getcomputedstyle" title="dom-Window-getComputedStyle">getComputedStyle</a>(<a class="external" data-anolis-spec="dom" href="https://dom.spec.whatwg.org/#element">Element</a> elt, optional DOMString pseudoElt);
+  --><a href="#dom-window-getcomputedstyle" title="dom-Window-getComputedStyle">getComputedStyle</a>(<a class="external" data-anolis-spec="dom" href="https://dom.spec.whatwg.org/#element">Element</a> elt, optional DOMString? pseudoElt);
 };</pre>
 
 <p>The <dfn id="dom-window-getcomputedstyle" title="dom-Window-getComputedStyle"><code>getComputedStyle(<var>elt</var>, <var>pseudoElt</var>)</code></dfn> method must run these

cssom/Overview.src.html

@@ -2882,7 +2882,7 @@ <h3>Extensions to the <code title>Window</code> Interface</h3>
 
 <pre class=idl>partial interface <span data-anolis-spec=html>Window</span> {
   [NewObject] <span>CSSStyleDeclaration</span> <!--
-  --><span title=dom-Window-getComputedStyle>getComputedStyle</span>(<span data-anolis-spec=dom>Element</span> elt, optional DOMString pseudoElt);
+  --><span title=dom-Window-getComputedStyle>getComputedStyle</span>(<span data-anolis-spec=dom>Element</span> elt, optional DOMString? pseudoElt);
 };</pre>
 
 <p>The <dfn title=dom-Window-getComputedStyle><code>getComputedStyle(<var>elt</var>, <var>pseudoElt</var>)</code></dfn> method must run these