Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/ui/webui/extensions/extension_settings_handler.cc

Index: chrome/browser/ui/webui/extensions/extension_settings_handler.cc
diff --git a/chrome/browser/ui/webui/extensions/extension_settings_handler.cc b/chrome/browser/ui/webui/extensions/extension_settings_handler.cc
index 902410dbd890d9e271d46e35501fe0b539bc63c9..262134c781653f4feb29b33ed74eb522e26fd5f3 100644
--- a/chrome/browser/ui/webui/extensions/extension_settings_handler.cc
+++ b/chrome/browser/ui/webui/extensions/extension_settings_handler.cc
@@ -444,7 +444,6 @@ void ExtensionSettingsHandler::Observe(
     case chrome::NOTIFICATION_EXTENSION_UPDATE_DISABLED:
     case chrome::NOTIFICATION_EXTENSION_WARNING_CHANGED:
     case chrome::NOTIFICATION_EXTENSION_BROWSER_ACTION_VISIBILITY_CHANGED:
-    case chrome::NOTIFICATION_PREF_CHANGED:
       MaybeUpdateAfterNotification();
       break;
     default:
@@ -452,6 +451,12 @@ void ExtensionSettingsHandler::Observe(
   }
 }
 
+void ExtensionSettingsHandler::OnPreferenceChanged(
+    PrefServiceBase* service,
+    const std::string& pref_name) {
+  MaybeUpdateAfterNotification();
+}
+
 void ExtensionSettingsHandler::ExtensionUninstallAccepted() {
   DCHECK(!extension_id_prompting_.empty());