Start counting at 1 for extension API validation.

chrome/renderer/resources/extension_process_bindings.js

Index: chrome/renderer/resources/extension_process_bindings.js
diff --git a/chrome/renderer/resources/extension_process_bindings.js b/chrome/renderer/resources/extension_process_bindings.js
index 43b6617500fc8db469037a3db7f34a8da423b10b..35d516e5db3f1f7b32ce008cb90cd522cd61a8b5 100644
--- a/chrome/renderer/resources/extension_process_bindings.js
+++ b/chrome/renderer/resources/extension_process_bindings.js
@@ -53,7 +53,7 @@ var chrome = chrome || {};
         if (validator.errors.length == 0)
           continue;
 
-        var message = "Invalid value for argument " + i + ". ";
+        var message = "Invalid value for argument " + (i + 1) + ". ";
         for (var i = 0, err; err = validator.errors[i]; i++) {
           if (err.path) {
             message += "Property '" + err.path + "': ";
@@ -67,7 +67,7 @@ var chrome = chrome || {};
 
         throw new Error(message);
       } else if (!schemas[i].optional) {
-        throw new Error("Parameter " + i + " is required.");
+        throw new Error("Parameter " + (i + 1) + " is required.");
       }
     }
   };