cros: Show dircryto migration banner

ui/login/account_picker/screen_account_picker.js

Index: ui/login/account_picker/screen_account_picker.js
diff --git a/ui/login/account_picker/screen_account_picker.js b/ui/login/account_picker/screen_account_picker.js
index 572404dfdd6dc4dedc3af414daadb228e75a6830..b2da10bd1ab6ce167724434184991057c36a18f5 100644
--- a/ui/login/account_picker/screen_account_picker.js
+++ b/ui/login/account_picker/screen_account_picker.js
@@ -388,12 +388,12 @@ login.createScreen('AccountPickerScreen', 'account-picker', function() {
      * Displays a banner containing |message|. If the banner is already present
      * this function updates the message in the banner. This function is used
      * by the chrome.screenlockPrivate.showMessage API.
-     * @param {string} message Text to be displayed
+     * @param {string} message Text to be displayed or empty to hide the banner.
      */
     showBannerMessage: function(message) {
       var banner = $('signin-banner');
       banner.textContent = message;
-      banner.classList.toggle('message-set', true);
+      banner.classList.toggle('message-set', !!message);
     },
 
     /**