[cssom] Make index argument optional in insertRule

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=27384

Author: zcorpan

cssom/Overview.bs

@@ -842,7 +842,7 @@ The {{CSSStyleSheet}} interface represents a <a>CSS style sheet</a>.
 interface CSSStyleSheet : StyleSheet {
   readonly attribute CSSRule? ownerRule;
   [SameObject] readonly attribute CSSRuleList cssRules;
-  unsigned long insertRule(DOMString rule, unsigned long index);
+  unsigned long insertRule(DOMString rule, optional unsigned long index = 0);
   void deleteRule(unsigned long index);
 };
 </pre>
@@ -1673,7 +1673,7 @@ The <code>CSSGroupingRule</code> interface represents an at-rule that contains o
 <pre class=idl>
 interface CSSGroupingRule : CSSRule {
   [SameObject] readonly attribute CSSRuleList cssRules;
-  unsigned long insertRule(DOMString rule, unsigned long index);
+  unsigned long insertRule(DOMString rule, optional unsigned long index = 0);
   void deleteRule(unsigned long index);
 };
 </pre>