[Offline Pages] Adding metadatas for namespace async_loading.

components/offline_pages/client_namespace_constants.cc

Index: components/offline_pages/client_namespace_constants.cc
diff --git a/components/offline_pages/client_namespace_constants.cc b/components/offline_pages/client_namespace_constants.cc
index 508db279a458988a0913974e85d0c453dbf90ab5..c6c67882c019e91a5d5759834b6cf197a52855a9 100644
--- a/components/offline_pages/client_namespace_constants.cc
+++ b/components/offline_pages/client_namespace_constants.cc
@@ -8,6 +8,7 @@ namespace offline_pages {
 
 const char kBookmarkNamespace[] = "bookmark";
 const char kLastNNamespace[] = "last_n";
+const char kAsyncNamespace[] = "async_loading";
 
 const char kDefaultNamespace[] = "default";