Add a SigninAllowed policy.

chrome/browser/ui/chrome_pages.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/chrome_pages.h"
 
 #include "base/logging.h"
 #include "base/stringprintf.h"
 #include "base/strings/string_number_conversions.h"
 #include "chrome/browser/download/download_shelf.h"
@@ skipping 182 matching lines @@
 
 void ShowSearchEngineSettings(Browser* browser) {
   content::RecordAction(UserMetricsAction("EditSearchEngines"));
   ShowSettingsSubPage(browser, kSearchEnginesSubPage);
 }
 
 void ShowBrowserSignin(Browser* browser, SyncPromoUI::Source source) {
   Profile* original_profile = browser->profile()->GetOriginalProfile();
   SigninManager* manager =
       SigninManagerFactory::GetForProfile(original_profile);
+  DCHECK(manager->IsSigninAllowed());
   // If we're signed in, just show settings.
   if (!manager->GetAuthenticatedUsername().empty()) {
     ShowSettings(browser);
   } else {
     // If the browser's profile is an incognito profile, make sure to use
     // a browser window from the original profile.  The user cannot sign in
     // from an incognito window.
     if (browser->profile()->IsOffTheRecord()) {
       browser =
           chrome::FindOrCreateTabbedBrowser(original_profile,
@@ skipping 30 matching lines @@
                     const std::string& service,
                     const GURL& continue_url) {
   GURL url(GaiaUrls::GetInstance()->service_login_url());
   url = net::AppendQueryParameter(url, "service", service);
   if (continue_url.is_valid())
     url = net::AppendQueryParameter(url, "continue", continue_url.spec());
   NavigateToSingletonTab(browser, url);
 }
 
 }  // namespace chrome