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 b8355edd0be73a281eca8e9978e1974d060c74c4..8c85e3ff753af33bb867dece8040a26158f4c410 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());