[cssom-view] Make HTMLElement#offsetParent nullable in IDL. https://www.w3.org/Bugs/Public/show_bug.cgi?id=22935

Author: Simon Pieters

cssom-view/Overview.html

@@ -1209,7 +1209,7 @@ <h3 id="element-scrolling-members"><span class="secno">7.2 </span><code title=""
 <h2 id="extensions-to-the-htmlelement-interface"><span class="secno">8 </span>Extensions to the <code title="">HTMLElement</code> Interface</h2>
 
 <pre class="idl">partial interface <a class="external" data-anolis-spec="html" href="http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html#htmlelement">HTMLElement</a> {
-  readonly attribute <a class="external" data-anolis-spec="dom" href="http://dom.spec.whatwg.org/#element">Element</a> <a href="#dom-htmlelement-offsetparent" title="dom-HTMLElement-offsetParent">offsetParent</a>;
+  readonly attribute <a class="external" data-anolis-spec="dom" href="http://dom.spec.whatwg.org/#element">Element</a>? <a href="#dom-htmlelement-offsetparent" title="dom-HTMLElement-offsetParent">offsetParent</a>;
   readonly attribute double <a href="#dom-htmlelement-offsettop" title="dom-HTMLElement-offsetTop">offsetTop</a>;
   readonly attribute double <a href="#dom-htmlelement-offsetleft" title="dom-HTMLElement-offsetLeft">offsetLeft</a>;
   readonly attribute double <a href="#dom-htmlelement-offsetwidth" title="dom-HTMLElement-offsetWidth">offsetWidth</a>;

cssom-view/Overview.src.html

@@ -1171,7 +1171,7 @@ <h3><code title>Element</code> Scrolling Members</h3>
 <h2>Extensions to the <code title>HTMLElement</code> Interface</h2>
 
 <pre class=idl>partial interface <span data-anolis-spec=html>HTMLElement</span> {
-  readonly attribute <span data-anolis-spec=dom>Element</span> <span title=dom-HTMLElement-offsetParent>offsetParent</span>;
+  readonly attribute <span data-anolis-spec=dom>Element</span>? <span title=dom-HTMLElement-offsetParent>offsetParent</span>;
   readonly attribute double <span title=dom-HTMLElement-offsetTop>offsetTop</span>;
   readonly attribute double <span title=dom-HTMLElement-offsetLeft>offsetLeft</span>;
   readonly attribute double <span title=dom-HTMLElement-offsetWidth>offsetWidth</span>;