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/string_number_conversions.h"
 #include "base/stringprintf.h"
 #include "chrome/browser/download/download_shelf.h"
@@ skipping 167 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);
+  // Check if Signin to Chrome is allowed by the policy.
+  if (!manager->IsSigninAllowed())

[sail, 2013/02/08 20:18:57]

Why would this code be reached if signin is not allowed.
If signin in was not allowed and the user did something to trigger this code
then what would the user see now?

[Adrian Kuegel, 2013/02/11 16:47:30]

Yes, I agree this code should never be reached if everything else works properly
(updating the corresponding command in BrowserCommandController for example).
If for example the command in the menu is not updated properly, and the user
clicks on it, nothing would happen.
As suggested by Andrew, I changed the code here into a DCHECK. But I can also
remove it if you want.

[sail, 2013/02/20 23:08:21]

DCHECK is fine.

+    return;
   // 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