Added Aura notification that relaunch and powerwash is required in case of downgrade.

chrome/browser/resources/help/help_page.js

 // Copyright 2014 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.
 
 cr.define('help', function() {
   var Page = cr.ui.pageManager.Page;
   var PageManager = cr.ui.pageManager.PageManager;
 
   /**
    * Encapsulated handling of the About page. Called 'help' internally to avoid
@@ skipping 34 matching lines @@
      * @private
      */
     status_: null,
 
     /**
      * Last message received from the version updater.
      * @private
      */
     message_: null,
 
+    /**
+     * True if user is allowed to change channels, false otherwise.
+     * @private
+     */
+    can_change_channel_: false,
+
     /** @override */
     initializePage: function() {
       Page.prototype.initializePage.call(this);
 
       $('product-license').innerHTML = loadTimeData.getString('productLicense');
       if (cr.isChromeOS) {
         $('product-os-license').innerHTML =
             loadTimeData.getString('productOsLicense');
       }
 
@@ skipping 243 matching lines @@
         }
       } else if (status == 'updated') {
         this.setUpdateImage_('up-to-date');
         $('update-status-message').innerHTML =
             loadTimeData.getString('upToDate');
       } else if (status == 'failed') {
         this.setUpdateImage_('failed');
         $('update-status-message').innerHTML = message;
       }
 
+      $('change-channel').disabled = !this.can_change_channel_ ||
+          status == 'nearly_updated';
+      $('channel-change-disallowed-icon').hidden = this.can_change_channel_;
+
       // Following invariant must be established at the end of this function:
       // { ~$('relaunch_and_powerwash').hidden -> $('relaunch').hidden }
       var relaunchAndPowerwashHidden = true;
       if ($('relaunch-and-powerwash')) {
         // It's allowed to do powerwash only for customer devices,
         // when user explicitly decides to update to a more stable
         // channel.
         relaunchAndPowerwashHidden =
             !this.targetChannelIsMoreStable_() || status != 'nearly_updated';
         $('relaunch-and-powerwash').hidden = relaunchAndPowerwashHidden;
@@ skipping 160 matching lines @@
       help.ChannelChangePage.updateTargetChannel(channel);
       this.updateUI_();
     },
 
     /**
      * @param {boolean} enabled True if the release channel can be enabled.
      * @private
      */
     updateEnableReleaseChannel_: function(enabled) {
       this.updateChannelChangerContainerVisibility_(enabled);
-      $('change-channel').disabled = !enabled;
-      $('channel-change-disallowed-icon').hidden = enabled;
+      this.can_change_channel_ = enabled;
+      this.updateUI_();
     },
 
     /**
      * Sets the device target channel.
      * @param {string} channel The name of the target channel.
      * @param {boolean} isPowerwashAllowed True iff powerwash is allowed.
      * @private
      */
     setChannel_: function(channel, isPowerwashAllowed) {
       chrome.send('setChannel', [channel, isPowerwashAllowed]);
@@ skipping 108 matching lines @@
 
   HelpPage.setBuildDate = function(buildDate) {
     HelpPage.getInstance().setBuildDate_(buildDate);
   };
 
   // Export
   return {
     HelpPage: HelpPage
   };
 });