Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/captive_portal/captive_portal_service.cc

Index: chrome/browser/captive_portal/captive_portal_service.cc
diff --git a/chrome/browser/captive_portal/captive_portal_service.cc b/chrome/browser/captive_portal/captive_portal_service.cc
index d6a013c268acf9a64545fcbebf5f8894966c5930..01ec60e0ca5b4ccda9e4337a9804fbe94bea21f8 100644
--- a/chrome/browser/captive_portal/captive_portal_service.cc
+++ b/chrome/browser/captive_portal/captive_portal_service.cc
@@ -271,14 +271,10 @@ void CaptivePortalService::OnPortalDetectionCompleted(
   OnResult(result);
 }
 
-void CaptivePortalService::Observe(
-    int type,
-    const content::NotificationSource& source,
-    const content::NotificationDetails& details) {
+void CaptivePortalService::OnPreferenceChanged(PrefServiceBase* service,
+                                               const std::string& pref_name) {
   DCHECK(CalledOnValidThread());
-  DCHECK_EQ(chrome::NOTIFICATION_PREF_CHANGED, type);
-  DCHECK_EQ(std::string(prefs::kAlternateErrorPagesEnabled),
-            *content::Details<std::string>(details).ptr());
+  DCHECK_EQ(prefs::kAlternateErrorPagesEnabled, pref_name);
   UpdateEnabledState();
 }