Make second argument to CSSStyleSheet.insertRule() optional again

Source/core/frame/UseCounter.cpp

Index: Source/core/frame/UseCounter.cpp
diff --git a/Source/core/frame/UseCounter.cpp b/Source/core/frame/UseCounter.cpp
index d0de31e1390ff731a1130981b4fda2f0a5c6a86d..1d97a31ece8bea6d662b20169622fe4bc61f89ac 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -682,6 +682,9 @@ String UseCounter::deprecationMessage(Feature feature)
     case ShowModalDialog:
         return "Chromium is considering deprecating showModalDialog. Please use window.open and postMessage instead.";
 
+    case CSSStyleSheetInsertRuleOptionalArg:
+        return "Calling CSSStyleSheet.insertRule() with one argument is deprecated. Please pass the index argument as well: insertRule(x, 0).";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();