Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/ui/webui/ntp/ntp_login_handler.cc

Index: chrome/browser/ui/webui/ntp/ntp_login_handler.cc
diff --git a/chrome/browser/ui/webui/ntp/ntp_login_handler.cc b/chrome/browser/ui/webui/ntp/ntp_login_handler.cc
index 4702fb1070521eb809cb5a82a7798b861ff664b4..476150a58fd297a610e04ebcce47122fff38f2a2 100644
--- a/chrome/browser/ui/webui/ntp/ntp_login_handler.cc
+++ b/chrome/browser/ui/webui/ntp/ntp_login_handler.cc
@@ -112,15 +112,17 @@ void NTPLoginHandler::Observe(int type,
                               const content::NotificationDetails& details) {
   if (type == chrome::NOTIFICATION_PROFILE_CACHED_INFO_CHANGED) {
     UpdateLogin();
-  } else if (type == chrome::NOTIFICATION_PREF_CHANGED) {
-    std::string* name = content::Details<std::string>(details).ptr();
-    if (prefs::kGoogleServicesUsername == *name)
-      UpdateLogin();
   } else {
     NOTREACHED();
   }
 }
 
+void NTPLoginHandler::OnPreferenceChanged(PrefServiceBase* service,
+                                          const std::string& pref_name) {
+  if (prefs::kGoogleServicesUsername == pref_name)
+    UpdateLogin();
+}
+
 void NTPLoginHandler::HandleInitializeSyncLogin(const ListValue* args) {
   UpdateLogin();
 }