Use RevocableStore to isolate the http cache from its transactions....

net/http/http_cache.h

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file declares a HttpTransactionFactory implementation that can be
 // layered on top of another HttpTransactionFactory to add HTTP caching.  The
 // caching logic follows RFC 2616 (any exceptions are called out in the code).
 //
 // The HttpCache takes a disk_cache::Backend as a parameter, and uses that for
 // the cache storage.
@@ skipping 146 matching lines @@
   ActiveEntriesSet doomed_entries_;
 
   ScopedRunnableMethodFactory<HttpCache> task_factory_;
 
   bool in_memory_cache_;
   int cache_size_;
 
   typedef base::hash_map<std::string, int> PlaybackCacheMap;
   scoped_ptr<PlaybackCacheMap> playback_cache_map_;
 
+  RevocableStore transactions_;
+
   DISALLOW_COPY_AND_ASSIGN(HttpCache);
 };
 
 }  // namespace net
 
 #endif  // NET_HTTP_HTTP_CACHE_H_