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 b2b2544220f96db5529078cbc010d8d2ba40ce53..c5f260b1f2a0a6614d98beecb1da73467b73f8d2 100644
--- a/chrome/browser/ui/webui/ntp/ntp_login_handler.cc
+++ b/chrome/browser/ui/webui/ntp/ntp_login_handler.cc
@@ -20,6 +20,8 @@
 #include "chrome/browser/profiles/profile_info_cache.h"
 #include "chrome/browser/profiles/profile_manager.h"
 #include "chrome/browser/profiles/profile_metrics.h"
+#include "chrome/browser/signin/signin_manager.h"
+#include "chrome/browser/signin/signin_manager_factory.h"
 #include "chrome/browser/sync/profile_sync_service.h"
 #include "chrome/browser/sync/profile_sync_service_factory.h"
 #include "chrome/browser/ui/browser.h"
@@ -91,6 +93,10 @@ 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());
@@ -252,7 +258,8 @@ bool NTPLoginHandler::ShouldShow(Profile* profile) {
   // UI and the avatar menu don't exist on that platform.
   return false;
 #else
-  return !profile->IsOffTheRecord();
+  SigninManager* signin = SigninManagerFactory::GetForProfile(profile);
+  return !profile->IsOffTheRecord() && signin && signin->IsSigninAllowed();
 #endif
 }