Bug 1358018. Make the second arg of CSSOM insertRule methods optional. r=heycam

This tracks the spec change in https://www.w3.org/Bugs/Public/show_bug.cgi?id=27384

dom/webidl/CSSGroupingRule.webidl

@@ -11,7 +11,7 @@
 interface CSSGroupingRule : CSSRule {
   [SameObject] readonly attribute CSSRuleList cssRules;
   [Throws]
-  unsigned long insertRule(DOMString rule, unsigned long index);
+  unsigned long insertRule(DOMString rule, optional unsigned long index = 0);
   [Throws]
   void deleteRule(unsigned long index);
 };

dom/webidl/CSSStyleSheet.webidl

@@ -21,7 +21,7 @@ interface CSSStyleSheet : StyleSheet {
   [ChromeOnly, BinaryName="parsingModeDOM"]
   readonly attribute CSSStyleSheetParsingMode parsingMode;
   [Throws, NeedsSubjectPrincipal]
-  unsigned long insertRule(DOMString rule, unsigned long index);
+  unsigned long insertRule(DOMString rule, optional unsigned long index = 0);
   [Throws, NeedsSubjectPrincipal]
   void deleteRule(unsigned long index);
 };