Add a SigninAllowed policy.

chrome/browser/ui/webui/ntp/ntp_login_handler.cc

Index: chrome/browser/ui/webui/ntp/ntp_login_handler.cc
diff --git a/chrome/browser/ui/webui/ntp/ntp_login_handler.cc b/chrome/browser/ui/webui/ntp/ntp_login_handler.cc
index ab788a1e6010a5d9ca332270efaa3f75ac4767f1..b7d06b3c1b5d2560ae60a454d1b7d999de4d622b 100644
--- a/chrome/browser/ui/webui/ntp/ntp_login_handler.cc
+++ b/chrome/browser/ui/webui/ntp/ntp_login_handler.cc
@@ -91,6 +91,10 @@ void NTPLoginHandler::RegisterMessages() {
                       pref_service,
                       base::Bind(&NTPLoginHandler::UpdateLogin,
                                  base::Unretained(this)));
+  signin_allowed_pref_.Init(prefs::kSigninAllowed,
+                            pref_service,
+                            base::Bind(&NTPLoginHandler::UpdateLogin,
+                                       base::Unretained(this)));
 
   registrar_.Add(this, chrome::NOTIFICATION_PROFILE_CACHED_INFO_CHANGED,
                  content::NotificationService::AllSources());
@@ -250,7 +254,7 @@ bool NTPLoginHandler::ShouldShow(Profile* profile) {
   // UI and the avatar menu don't exist on that platform.
   return false;
 #else
-  return !profile->IsOffTheRecord();
+  return !profile->IsOffTheRecord() && profile->IsSigninAllowed();
 #endif
 }