Remove NOTIFICATION_SYSTEM_SETTING_CHANGED, switch CrosSettings to base::CallbackRegistry.

chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc

Index: chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
diff --git a/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc b/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
index 56614f2b9881026669c381489d1b65284ad2fc79..497945c120b28e38a598f592d65da024faab6795 100644
--- a/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
+++ b/chrome/browser/ui/webui/chromeos/login/signin_screen_handler.cc
@@ -403,8 +403,14 @@ SigninScreenHandler::SigninScreenHandler(
   DCHECK(error_screen_actor_);
   DCHECK(core_oobe_actor_);
   network_state_informer_->AddObserver(this);
-  CrosSettings::Get()->AddSettingsObserver(kAccountsPrefAllowNewUser, this);
-  CrosSettings::Get()->AddSettingsObserver(kAccountsPrefAllowGuest, this);
+  allow_new_user_subscription_ = CrosSettings::Get()->AddSettingsObserver(
+      kAccountsPrefAllowNewUser,
+      base::Bind(&SigninScreenHandler::UserSettingsChanged,
+                 base::Unretained(this)));
+  allow_guest_subscription_ = CrosSettings::Get()->AddSettingsObserver(
+      kAccountsPrefAllowGuest,
+      base::Bind(&SigninScreenHandler::UserSettingsChanged,
+                 base::Unretained(this)));
 
   registrar_.Add(this,
                  chrome::NOTIFICATION_AUTH_NEEDED,
@@ -426,8 +432,6 @@ SigninScreenHandler::~SigninScreenHandler() {
   if (delegate_)
     delegate_->SetWebUIHandler(NULL);
   network_state_informer_->RemoveObserver(this);
-  CrosSettings::Get()->RemoveSettingsObserver(kAccountsPrefAllowNewUser, this);
-  CrosSettings::Get()->RemoveSettingsObserver(kAccountsPrefAllowGuest, this);
 }
 
 void SigninScreenHandler::DeclareLocalizedValues(
@@ -993,11 +997,6 @@ void SigninScreenHandler::Observe(int type,
                                   const content::NotificationSource& source,
                                   const content::NotificationDetails& details) {
   switch (type) {
-    case chrome::NOTIFICATION_SYSTEM_SETTING_CHANGED: {
-      UpdateAuthExtension();
-      UpdateAddButtonStatus();
-      break;
-    }
     case chrome::NOTIFICATION_AUTH_NEEDED: {
       has_pending_auth_ui_ = true;
       break;
@@ -1181,6 +1180,11 @@ void SigninScreenHandler::LoadAuthExtension(
   CallJS("login.GaiaSigninScreen.loadAuthExtension", params);
 }
 
+void SigninScreenHandler::UserSettingsChanged() {
+  UpdateAuthExtension();
+  UpdateAddButtonStatus();
+}
+
 void SigninScreenHandler::UpdateAuthExtension() {
   DictionaryValue params;
   UpdateAuthParams(&params);