About page UI: add confirmation message for release channel switch.

chrome/browser/resources/help/help.js

Index: chrome/browser/resources/help/help.js
diff --git a/chrome/browser/resources/help/help.js b/chrome/browser/resources/help/help.js
index a6bbf3333f8986d93fab5aa4ac2ee9350f248e29..cbc6a3335c4be54db8d81053b7470fccb4c26112 100644
--- a/chrome/browser/resources/help/help.js
+++ b/chrome/browser/resources/help/help.js
@@ -52,6 +52,12 @@ cr.define('help', function() {
         chrome.send('relaunchNow');
       });
 
+      this.channelName_ = {
+          'stable-channel': loadTimeData.getString('stable'),
+          'beta-channel': loadTimeData.getString('beta'),
+          'dev-channel': loadTimeData.getString('dev')
+      };
+
       var channelChanger = $('channel-changer');
       if (channelChanger) {
         var self = this;
@@ -215,6 +221,9 @@ cr.define('help', function() {
      */
     setReleaseChannel_: function(channel) {
       chrome.send('setReleaseTrack', [channel]);
+      $('channel-change-confirmation').hidden = false;
+      $('channel-change-confirmation').textContent =
+        loadTimeData.getStringF('channel-changed', this.channelName_[channel]);
     },
 
     /**