Add a SigninAllowed policy.

chrome/browser/signin/signin_ui_util.cc

Index: chrome/browser/signin/signin_ui_util.cc
diff --git a/chrome/browser/signin/signin_ui_util.cc b/chrome/browser/signin/signin_ui_util.cc
index 61bd7306057c6fed97fbada9f8b148c6cae19f6c..20374e283caafd0220fd9e6b70585fb47f0244d5 100644
--- a/chrome/browser/signin/signin_ui_util.cc
+++ b/chrome/browser/signin/signin_ui_util.cc
@@ -54,8 +54,9 @@ string16 GetSigninMenuLabel(Profile* profile) {
     return error->MenuItemLabel();
 
   // No errors, so just display the signed in user, if any.
-  ProfileSyncService* service = profile->IsSyncAccessible() ?
-      ProfileSyncServiceFactory::GetForProfile(profile) : NULL;
+  ProfileSyncService* service =
+      profile->IsSyncAccessible() ?

[Andrew T Wilson (Slow), 2013/02/07 10:08:11]

This formatting change is fine, but you might want to revert this file to avoid
churn in the revision log.

[Adrian Kuegel, 2013/02/07 11:09:21]

I should have checked that I reformat it the same way it was before. This was
not an intended change, and I reverted the file.

+          ProfileSyncServiceFactory::GetForProfile(profile) : NULL;
 
   // Even if the user is signed in, don't display the "signed in as..."
   // label if we're still setting up sync.