Introduce AppCacheResponseMetadataWriter to support metadata caching.

content/browser/appcache/mock_appcache_storage.cc

Index: content/browser/appcache/mock_appcache_storage.cc
diff --git a/content/browser/appcache/mock_appcache_storage.cc b/content/browser/appcache/mock_appcache_storage.cc
index 91630f7d351ef9e41a2fbd9c65244791a71a0dec..2a337a0e4a2b7c4ad87ca183dc43b1ae68474ce9 100644
--- a/content/browser/appcache/mock_appcache_storage.cc
+++ b/content/browser/appcache/mock_appcache_storage.cc
@@ -165,6 +165,13 @@ AppCacheResponseWriter* MockAppCacheStorage::CreateResponseWriter(
   return new AppCacheResponseWriter(NewResponseId(),  group_id, disk_cache());
 }
 
+AppCacheResponseMetadataWriter*
+MockAppCacheStorage::CreateResponseMetadataWriter(int64 group_id,
+                                                  int64 response_id) {
+  return new AppCacheResponseMetadataWriter(response_id, group_id,
+                                            disk_cache());
+}
+
 void MockAppCacheStorage::DoomResponses(
     const GURL& manifest_url, const std::vector<int64>& response_ids) {
   DeleteResponses(manifest_url, response_ids);