Start counting at 1 for extension API validation.

chrome/renderer/resources/extension_process_bindings.js

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This script contains privileged chrome extension related javascript APIs.
 // It is loaded by pages whose URL has the chrome-extension protocol.
 
 var chrome = chrome || {};
 (function() {
   native function GetExtensionAPIDefinition();
@@ skipping 35 matching lines @@
       throw new Error("Too many arguments.");
 
     for (var i = 0; i < schemas.length; i++) {
       if (i in args && args[i] !== null && args[i] !== undefined) {
         var validator = new chromeHidden.JSONSchemaValidator();
         validator.addTypes(chromeHidden.validationTypes);
         validator.validate(args[i], schemas[i]);
         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 + "': ";
           }
           message += err.message;
           message = message.substring(0, message.length - 1);
           message += ", ";
         }
         message = message.substring(0, message.length - 2);
         message += ".";
 
         throw new Error(message);
       } else if (!schemas[i].optional) {
-        throw new Error("Parameter " + i + " is required.");
+        throw new Error("Parameter " + (i + 1) + " is required.");
       }
     }
   };
 
   // Callback handling.
   var requests = [];
   chromeHidden.handleResponse = function(requestId, name,
                                          success, response, error) {
     try {
       var request = requests[requestId];
@@ skipping 721 matching lines @@
 
   if (!chrome.experimental)
     chrome.experimental = {};
 
   if (!chrome.experimental.accessibility)
     chrome.experimental.accessibility = {};
 
   if (!chrome.experimental.tts)
     chrome.experimental.tts = {};
 })();