Improving `content_security_policy` documentation.

chrome/common/extensions/docs/examples/tutorials/analytics/popup.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.
+
+/**
+ * Add your Analytics tracking ID here.
+ */
+var _AnalyticsCode = 'UA-XXXXXX-X';
+
+/**
+ * Below is a modified version of the Google Analytics asynchronous tracking
+ * code snippet.  It has been modified to pull the HTTPS version of ga.js
+ * instead of the default HTTP version.  It is recommended that you use this
+ * snippet instead of the standard tracking snippet provided when setting up
+ * a Google Analytics account.
+ */
+var _gaq = _gaq || [];
+_gaq.push(['_setAccount', _AnalyticsCode]);
+_gaq.push(['_trackPageview']);
+
+(function() {
+  var ga = document.createElement('script');
+  ga.type = 'text/javascript';
+  ga.async = true;
+  ga.src = 'https://ssl.google-analytics.com/ga.js';
+  var s = document.getElementsByTagName('script')[0];
+  s.parentNode.insertBefore(ga, s);
+})();
+
+/**
+ * Track a click on a button using the asynchronous tracking API.
+ *
+ * See http://code.google.com/apis/analytics/docs/tracking/asyncTracking.html
+ * for information on how to use the asynchronous tracking API.
+ */
+function trackButtonClick(e) {
+  _gaq.push(['_trackEvent', e.target.id, 'clicked']);
+}
+
+/**
+ * Now set up your event handlers for the popup's `button` elements once the
+ * popup's DOM has loaded.
+ */
+document.addEventListener('DOMContentLoaded', function () {
+  var buttons = document.querySelectorAll('button');
+  for (var i = 0; i < buttons.length; i++) {
+    buttons[i].addEventListener('click', trackButtonClick);
+  }
+});