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

chrome/common/extensions/docs/examples/api/contextMenus/event_page/sample.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.
 
 // The onClicked callback function.
 function onClickHandler(info, tab) {
   if (info.menuItemId == "radio1" || info.menuItemId == "radio2") {
     console.log("radio item " + info.menuItemId +
                 " was clicked (previous checked state was "  +
                 info.wasChecked + ")");
@@ skipping 45 matching lines @@
       {"title": "Checkbox1", "type": "checkbox", "id": "checkbox1"});
   chrome.contextMenus.create(
       {"title": "Checkbox2", "type": "checkbox", "id": "checkbox2"});
   console.log("checkbox1 checkbox2");
 
   // Intentionally create an invalid item, to show off error checking in the
   // create callback.
   console.log("About to try creating an invalid item - an error about " +
       "duplicate item child1 should show up");
   chrome.contextMenus.create({"title": "Oops", "id": "child1"}, function() {
-    if (chrome.extension.lastError) {
-      console.log("Got expected error: " + chrome.extension.lastError.message);
+    if (chrome.runtime.lastError) {
+      console.log("Got expected error: " + chrome.runtime.lastError.message);
     }
   });
 });