Introduce AppCacheResponseMetadataWriter to support metadata caching.

content/browser/appcache/appcache_storage_impl.cc

Index: content/browser/appcache/appcache_storage_impl.cc
diff --git a/content/browser/appcache/appcache_storage_impl.cc b/content/browser/appcache/appcache_storage_impl.cc
index 5df1d3ebdd15055a11c3b757d14a9070cdbf5f14..4bbb03ab9e3130dd15593e3b129fbb76f2491897 100644
--- a/content/browser/appcache/appcache_storage_impl.cc
+++ b/content/browser/appcache/appcache_storage_impl.cc
@@ -1643,6 +1643,13 @@ AppCacheResponseWriter* AppCacheStorageImpl::CreateResponseWriter(
   return new AppCacheResponseWriter(NewResponseId(), group_id, disk_cache());
 }
 
+AppCacheResponseMetadataWriter*
+AppCacheStorageImpl::CreateResponseMetadataWriter(int64 group_id,
+                                                  int64 response_id) {
+  return new AppCacheResponseMetadataWriter(response_id, group_id,
+                                            disk_cache());
+}
+
 void AppCacheStorageImpl::DoomResponses(
     const GURL& manifest_url, const std::vector<int64>& response_ids) {
   if (response_ids.empty())