Update references to the extension messaging APIs to point to the "runtime"

chrome/common/extensions/docs/examples/api/debugger/live-headers/background.js

 // Copyright (c) 2012 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.
 
 chrome.browserAction.onClicked.addListener(function() {
   chrome.windows.getCurrent(function(win) {
     chrome.tabs.getSelected(win.id, actionClicked);
   });
 });
 
 var version = "1.0";
 
 function actionClicked(tab) {
   chrome.debugger.attach({tabId:tab.id}, version, onAttach.bind(null, tab.id));
 }
 
 function onAttach(tabId) {
-  if (chrome.extension.lastError) {
-    alert(chrome.extension.lastError.message);
+  if (chrome.runtime.lastError) {
+    alert(chrome.runtime.lastError.message);
     return;
   }
 
   chrome.windows.create(
       {url: "headers.html?" + tabId, type: "popup", width: 800, height: 600});
 }