Remove NOTIFICATION_SYSTEM_SETTING_CHANGED, switch CrosSettings to base::CallbackRegistry.

chrome/browser/chromeos/login/existing_user_controller.h

Index: chrome/browser/chromeos/login/existing_user_controller.h
diff --git a/chrome/browser/chromeos/login/existing_user_controller.h b/chrome/browser/chromeos/login/existing_user_controller.h
index dde8e6e11764a1c52af782325b55ab993c5d44d2..1440a063e61655617992e88d29a2074db162354a 100644
--- a/chrome/browser/chromeos/login/existing_user_controller.h
+++ b/chrome/browser/chromeos/login/existing_user_controller.h
@@ -21,6 +21,7 @@
 #include "chrome/browser/chromeos/login/login_performer.h"
 #include "chrome/browser/chromeos/login/login_utils.h"
 #include "chrome/browser/chromeos/login/user.h"
+#include "chrome/browser/chromeos/settings/cros_settings.h"
 #include "chrome/browser/chromeos/settings/device_settings_service.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
@@ -147,6 +148,9 @@ class ExistingUserController : public LoginDisplay::Delegate,
   // LoginUtils::Delegate implementation:
   virtual void OnProfilePrepared(Profile* profile) OVERRIDE;
 
+  // Called when the settings change.

[Mattias Nissler (ping if slow), 2013/09/18 09:09:48]

For clarity, rather "Called when device settings change.". Can you also rename
SettingsChanged() to DeviceSettingsChanged()?

[Avi (use Gerrit), 2013/09/18 16:41:44]

Done.

+  void SettingsChanged();
+
   // Starts WizardController with the specified screen.
   void ActivateWizard(const std::string& screen_name);
 
@@ -298,6 +302,15 @@ class ExistingUserController : public LoginDisplay::Delegate,
 
   scoped_ptr<login::NetworkStateHelper> network_state_helper_;
 
+  scoped_ptr<CrosSettings::ObserverSubscription> show_user_names_subscription_;
+  scoped_ptr<CrosSettings::ObserverSubscription> allow_new_user_subscription_;
+  scoped_ptr<CrosSettings::ObserverSubscription> allow_guest_subscription_;
+  scoped_ptr<CrosSettings::ObserverSubscription> users_subscription_;
+  scoped_ptr<CrosSettings::ObserverSubscription>
+      local_account_auto_login_id_subscription_;
+  scoped_ptr<CrosSettings::ObserverSubscription>
+      local_account_auto_login_delay_subscription_;

[Mattias Nissler (ping if slow), 2013/09/18 09:09:48]

This is quite some boilerplate and lots of small heap-allocated objects. I
assume this has thoroughly been discussed and measured and found to be
acceptable if the entire code base were to be converted to this style?

[Avi (use Gerrit), 2013/09/18 16:41:44]

If you want I can bring this up as a specific example on cr-dev for discussion.

+
   FRIEND_TEST_ALL_PREFIXES(ExistingUserControllerTest, ExistingUserLogin);
 
   DISALLOW_COPY_AND_ASSIGN(ExistingUserController);