Add a SigninAllowed policy.

chrome/browser/ui/chrome_pages.cc

Index: chrome/browser/ui/chrome_pages.cc
diff --git a/chrome/browser/ui/chrome_pages.cc b/chrome/browser/ui/chrome_pages.cc
index 9255140465d05bc515f18c0bde5b6497c8354931..fe73c53b1ccc723e01fa1a7eded1b5ef34a98727 100644
--- a/chrome/browser/ui/chrome_pages.cc
+++ b/chrome/browser/ui/chrome_pages.cc
@@ -10,6 +10,7 @@
 #include "chrome/browser/download/download_shelf.h"
 #include "chrome/browser/profiles/profile.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"
@@ -184,6 +185,10 @@ void ShowSearchEngineSettings(Browser* browser) {
 
 void ShowBrowserSignin(Browser* browser, SyncPromoUI::Source source) {
   Profile* original_profile = browser->profile()->GetOriginalProfile();
+  // Check if Signin to Chrome is allowed by the policy.
+  SigninManager* signin = SigninManagerFactory::GetForProfile(original_profile);
+  if (!signin->IsSigninAllowed())
+    return;
   ProfileSyncService* service =
       ProfileSyncServiceFactory::GetInstance()->GetForProfile(
           original_profile);