Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/profiles/gaia_info_update_service.cc

Index: chrome/browser/profiles/gaia_info_update_service.cc
diff --git a/chrome/browser/profiles/gaia_info_update_service.cc b/chrome/browser/profiles/gaia_info_update_service.cc
index efd51885f85c0e2abb42af3c0026400c493257f7..4d8b4a8935c26d056b45ac46e2d4cb5bdc7f614e 100644
--- a/chrome/browser/profiles/gaia_info_update_service.cc
+++ b/chrome/browser/profiles/gaia_info_update_service.cc
@@ -164,17 +164,10 @@ void GAIAInfoUpdateService::OnProfileDownloadFailure(
   ScheduleNextUpdate();
 }
 
-void GAIAInfoUpdateService::Observe(
-    int type,
-    const content::NotificationSource& source,
-    const content::NotificationDetails& details) {
-  if (type == chrome::NOTIFICATION_PREF_CHANGED) {
-    std::string* name = content::Details<std::string>(details).ptr();
-    if (prefs::kGoogleServicesUsername == *name)
-      OnUsernameChanged();
-  } else {
-    NOTREACHED();
-  }
+void GAIAInfoUpdateService::OnPreferenceChanged(PrefServiceBase* service,
+                                                const std::string& pref_name) {
+  if (prefs::kGoogleServicesUsername == pref_name)
+    OnUsernameChanged();
 }
 
 void GAIAInfoUpdateService::OnUsernameChanged() {