[cssom] Change IDL [ArrayClass] to [LegacyArrayClass]

Fixes w3c#424.

Author: zcorpan

cssom/Overview.bs

@@ -447,7 +447,7 @@ The {{MediaList}} Interface {#the-medialist-interface}
 An object that implements the <code>MediaList</code> interface has an associated <dfn export for=MediaList>collection of media queries</dfn>.
 
 <pre class=idl>
-[ArrayClass]
+[LegacyArrayClass]
 interface MediaList {
   [TreatNullAs=EmptyString] stringifier attribute DOMString mediaText;
   readonly attribute unsigned long length;
@@ -1062,7 +1062,7 @@ value of the header.
 The {{StyleSheetList}} interface represents an ordered collection of <a>CSS style sheets</a>.
 
 <pre class=idl>
-[ArrayClass]
+[LegacyArrayClass]
 interface StyleSheetList {
   getter StyleSheet? item(unsigned long index);
   readonly attribute unsigned long length;
@@ -1515,7 +1515,7 @@ To <dfn>remove a CSS rule</dfn> from a CSS rule list <var>list</var> at index <v
 The {{CSSRuleList}} interface represents an ordered collection of CSS style rules.
 
 <pre class=idl>
-[ArrayClass]
+[LegacyArrayClass]
 interface CSSRuleList {
   getter CSSRule? item(unsigned long index);
   readonly attribute unsigned long length;