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

chrome/common/extensions/docs/examples/api/eventPage/basic/content.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.
 
 document.body.innerHTML = "";
 
 function addButton(name, cb) {
   var a = document.createElement("button");
   a.innerText = name;
   a.onclick = cb;
   document.body.appendChild(document.createElement("br"));
   document.body.appendChild(a);
 }
 
 function log(str) {
   console.log(str);
   logDiv.innerHTML += str + "<br>";
 }
 
 addButton("Clear logs", function() {
   logDiv.innerHTML = "";
 });
 
 addButton("Send message with delayed response", function() {
-  chrome.extension.sendMessage({delayedResponse: true}, function(response) {
+  chrome.runtime.sendMessage({delayedResponse: true}, function(response) {
     log("Background page responded: " + response);
   });
 });
 
 addButton("Show counters", function() {
-  chrome.extension.sendMessage({getCounters: true}, function(response) {
+  chrome.runtime.sendMessage({getCounters: true}, function(response) {
     log("In-memory counter is: " + response.counter);
     log("Persisted counter is: " + response.persistentCounter);
   });
 });
 
 addButton("Set an alarm", function() {
-  chrome.extension.sendMessage({setAlarm: true});
+  chrome.runtime.sendMessage({setAlarm: true});
 });
 
-chrome.extension.onMessage.addListener(function(msg, _, sendResponse) {
+chrome.runtime.onMessage.addListener(function(msg, _, sendResponse) {
   log("Got message from background page: " + msg);
 });
 
 var logDiv = document.createElement("div");
 logDiv.style.border = "1px dashed black";
 document.body.appendChild(document.createElement("br"));
 document.body.appendChild(logDiv);
 
 log("Ready.");