Hide the process of instantiating NotificationServiceImpl from embedders.

content/browser/notification_service_impl.cc

Index: content/browser/notification_service_impl.cc
diff --git a/content/browser/notification_service_impl.cc b/content/browser/notification_service_impl.cc
index 221a883477277636954df7e550eb361305a83775..327e28fba5df2a221043966238832576ccb0d4d2 100644
--- a/content/browser/notification_service_impl.cc
+++ b/content/browser/notification_service_impl.cc
@@ -23,6 +23,11 @@ content::NotificationService* content::NotificationService::current() {
 }
 
 // static
+content::NotificationService* content::NotificationService::Create() {
+  return new NotificationServiceImpl;
+}
+
+// static
 bool NotificationServiceImpl::HasKey(const NotificationSourceMap& map,
                                  const content::NotificationSource& source) {
   return map.find(source.map_key()) != map.end();