Introduce AppCacheResponseMetadataWriter to support metadata caching.

content/browser/appcache/appcache_storage.h

Index: content/browser/appcache/appcache_storage.h
diff --git a/content/browser/appcache/appcache_storage.h b/content/browser/appcache/appcache_storage.h
index edc059b9381f894ac202dd44ddff063cd328d6ca..854b0d1da4782567798630e6b129147eb4d5b75f 100644
--- a/content/browser/appcache/appcache_storage.h
+++ b/content/browser/appcache/appcache_storage.h
@@ -26,6 +26,7 @@ class AppCache;
 class AppCacheEntry;
 class AppCacheGroup;
 class AppCacheQuotaClientTest;
+class AppCacheResponseMetadataWriter;
 class AppCacheResponseReader;
 class AppCacheResponseTest;
 class AppCacheResponseWriter;
@@ -169,6 +170,11 @@ class CONTENT_EXPORT AppCacheStorage {
   virtual AppCacheResponseWriter* CreateResponseWriter(
       const GURL& manifest_url, int64 group_id) = 0;
 
+  // Creates a metadata writer to write metadata of response to storage.
+  virtual AppCacheResponseMetadataWriter* CreateResponseMetadataWriter(
+      int64 group_id,
+      int64 response_id) = 0;
+
   // Schedules the lazy deletion of responses and saves the ids
   // persistently such that the responses will be deleted upon restart
   // if they aren't deleted prior to shutdown.