Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/safe_browsing/safe_browsing_service.cc

Index: chrome/browser/safe_browsing/safe_browsing_service.cc
diff --git a/chrome/browser/safe_browsing/safe_browsing_service.cc b/chrome/browser/safe_browsing/safe_browsing_service.cc
index 53df55a93029bdb9fb57a37f9c17a49570769d40..fbfd8d114ce74d6cf09129296eb94b201c99160f 100644
--- a/chrome/browser/safe_browsing/safe_browsing_service.cc
+++ b/chrome/browser/safe_browsing/safe_browsing_service.cc
@@ -1364,18 +1364,18 @@ void SafeBrowsingService::Observe(int type,
         RemovePrefService(profile->GetPrefs());
       break;
     }
-    case chrome::NOTIFICATION_PREF_CHANGED: {
-      DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
-      std::string* pref = content::Details<std::string>(details).ptr();
-      DCHECK(*pref == prefs::kSafeBrowsingEnabled);
-      RefreshState();
-      break;
-    }
     default:
       NOTREACHED();
   }
 }
 
+void SafeBrowsingService::OnPreferenceChanged(PrefServiceBase* service,
+                                              const std::string& pref_name) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(pref_name == prefs::kSafeBrowsingEnabled);
+  RefreshState();
+}
+
 bool SafeBrowsingService::IsWhitelisted(const UnsafeResource& resource) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   // Check if the user has already ignored our warning for this render_view