[cssom] Switch from DOMString[] to FrozenArray<DOMString>

Fixes http://www.w3.org/mid/55A7D8C4.8030709@mit.edu.

Author: zcorpan

cssom/Overview.bs

@@ -1159,7 +1159,7 @@ partial interface Document {
   attribute DOMString? selectedStyleSheetSet;
   readonly attribute DOMString? lastStyleSheetSet;
   readonly attribute DOMString? preferredStyleSheetSet;
-  readonly attribute DOMString[] styleSheetSets;
+  readonly attribute FrozenArray<DOMString> styleSheetSets;
   void enableStyleSheetsForSet(DOMString? name);
 };
 </pre>
@@ -1221,7 +1221,7 @@ name</a>.
 Note: Unlike {{Document/lastStyleSheetSet}},
 this attribute is initially the empty string.
 
-The <dfn attribute for=Document>styleSheetSets</dfn> attribute must return a read only array of the
+The <dfn attribute for=Document>styleSheetSets</dfn> attribute must return a frozen array of the
 <a lt="CSS style sheet set name">CSS style sheet set names</a> of the <a lt="CSS style sheet set">CSS style sheet sets</a>, in order of the
 <a>document CSS style sheets</a>. The array is live; if the <a>document CSS style sheets</a> change, the array must be updated as appropriate.