Add a SigninAllowed policy.

chrome/browser/ui/browser_command_controller.h

Index: chrome/browser/ui/browser_command_controller.h
diff --git a/chrome/browser/ui/browser_command_controller.h b/chrome/browser/ui/browser_command_controller.h
index e9fa6275385f66bd79c310c82b16da609215c402..13d06238b95870f161aa07c474ad3dd07b06e04a 100644
--- a/chrome/browser/ui/browser_command_controller.h
+++ b/chrome/browser/ui/browser_command_controller.h
@@ -6,11 +6,11 @@
 #define CHROME_BROWSER_UI_BROWSER_COMMAND_CONTROLLER_H_
 
 #include "base/prefs/public/pref_change_registrar.h"
-#include "chrome/browser/api/sync/profile_sync_service_observer.h"
 #include "chrome/browser/command_updater.h"
 #include "chrome/browser/command_updater_delegate.h"
 #include "chrome/browser/profiles/profile_info_cache_observer.h"
 #include "chrome/browser/sessions/tab_restore_service_observer.h"
+#include "chrome/browser/signin/signin_pref_observer.h"
 #include "chrome/browser/ui/tabs/tab_strip_model_observer.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
@@ -30,6 +30,7 @@ namespace chrome {
 class BrowserCommandController : public CommandUpdaterDelegate,
                                  public content::NotificationObserver,
                                  public ProfileInfoCacheObserver,
+                                 public SigninPrefObserver,
                                  public TabStripModelObserver,
                                  public TabRestoreServiceObserver {
  public:
@@ -127,6 +128,9 @@ class BrowserCommandController : public CommandUpdaterDelegate,
   virtual void TabRestoreServiceChanged(TabRestoreService* service) OVERRIDE;
   virtual void TabRestoreServiceDestroyed(TabRestoreService* service) OVERRIDE;
 
+  // Overridden from SigninPrefObserver:
+  virtual void OnSigninAllowedPrefChange(bool signin_allowed) OVERRIDE;
+
   // Returns true if the regular Chrome UI (not the fullscreen one and
   // not the single-tab one) is shown. Used for updating window command states
   // only. Consider using SupportsWindowFeature if you need the mentioned