[cssom] Define what kind of array styleSheetSets returns. https://www.w3.org/Bugs/Public/show_bug.cgi?id=16111

Author: Simon Pieters

cssom/Overview.html

@@ -1187,7 +1187,7 @@ <h4 id="extensions-to-the-document-interface"><span class="secno">6.2.3 </span>E
 <dt class="attribute" id="widl-Document-styleSheetSets">
 <dfn id="widl-Document-styleSheetSets"><code>styleSheetSets</code></dfn> of type <span class="idlAttrType"><a>DOMString</a>[]</span>, readonly</dt>
 <dd>
-<p>The <code><a href="#widl-Document-styleSheetSets">styleSheetSets</a></code> attribute must  return a list of the <a href="#style-sheet-set-name" title="style sheet set name">style    sheet set names</a> of the <a href="#style-sheet-set" title="style sheet set">style sheet sets</a>, in order    of the <a href="#document-style-sheets">document style sheets</a>.</dd>
+<p>The <code><a href="#widl-Document-styleSheetSets">styleSheetSets</a></code> attribute must return a read only array of the    <a href="#style-sheet-set-name" title="style sheet set name">style sheet set names</a> of the    <a href="#style-sheet-set" title="style sheet set">style sheet sets</a>, in order of the <a href="#document-style-sheets">document    style sheets</a>.</dd>
 </dl>
 <dl class="methods">
 <dt class="method" id="widl-Document-enableStyleSheetsForSet-void-DOMString-name">

cssom/idl/Document.idl

@@ -115,9 +115,10 @@ partial interface Document {
   readonly attribute DOMString? preferredStyleSheetSet;         // new
 
   [Documentation=
-   "<p>The {@name} {@type} must  return a list of the <span title='style sheet set name'>style\
-    sheet set names</span> of the <span title='style sheet set'>style sheet sets</span>, in order\
-    of the <span>document style sheets</span>.</p>"
+   "<p>The {@name} {@type} must return a read only array of the\
+    <span title='style sheet set name'>style sheet set names</span> of the\
+    <span title='style sheet set'>style sheet sets</span>, in order of the <span>document\
+    style sheets</span>.</p>"
   ]
   readonly attribute DOMString[] styleSheetSets;                // new