[cssom] Remove [LegacyArrayClass] from CSSOM interfaces.

Blink and WebKit don't really support it in these interfaces either, and I'm
removing the remaining usage in Gecko for MediaList and updating tests in
https://bugzil.la/1456256.

Fixes w3c#2601

Author: emilio

cssom-1/Overview.bs

@@ -503,8 +503,7 @@ 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>
-[Exposed=Window,
- LegacyArrayClass]
+[Exposed=Window]
 interface MediaList {
   stringifier attribute [TreatNullAs=EmptyString] CSSOMString mediaText;
   readonly attribute unsigned long length;
@@ -1122,8 +1121,7 @@ value of the header.
 The {{StyleSheetList}} interface represents an ordered collection of <a>CSS style sheets</a>.
 
 <pre class=idl>
-[Exposed=Window,
- LegacyArrayClass]
+[Exposed=Window]
 interface StyleSheetList {
   getter StyleSheet? item(unsigned long index);
   readonly attribute unsigned long length;
@@ -1639,8 +1637,7 @@ To <dfn export>remove a CSS rule</dfn> from a CSS rule list <var>list</var> at i
 The {{CSSRuleList}} interface represents an ordered collection of CSS style rules.
 
 <pre class=idl>
-[Exposed=Window,
- LegacyArrayClass]
+[Exposed=Window]
 interface CSSRuleList {
   getter CSSRule? item(unsigned long index);
   readonly attribute unsigned long length;