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..983b2fa381ed3c59f556953b1ec734960d2e1d96 100644
--- a/chrome/browser/resources/help/help.js
+++ b/chrome/browser/resources/help/help.js
@@ -54,6 +54,11 @@ cr.define('help', function() {
 
       var channelChanger = $('channel-changer');
       if (channelChanger) {
+        this.channelName_ = {
+            'stable-channel': loadTimeData.getString('stable'),
+            'beta-channel': loadTimeData.getString('beta'),
+            'dev-channel': loadTimeData.getString('dev')
+        };
         var self = this;
         channelChanger.onchange = function(event) {
           self.setReleaseChannel_(event.target.value);
@@ -215,6 +220,9 @@ 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]);
     },
 
     /**