RFC: use some in-memory state in SimpleCache to quickly cache-miss some CantConditionalize cases

net/disk_cache/simple/simple_backend_version.h

 // Copyright 2013 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.
 
 #ifndef NET_DISK_CACHE_SIMPLE_SIMPLE_BACKEND_VERSION_H_
 #define NET_DISK_CACHE_SIMPLE_SIMPLE_BACKEND_VERSION_H_
 
 #include <stdint.h>
 
 namespace disk_cache {
 
 // Short rules helping to think about data upgrades within Simple Cache:
 //   * ALL changes of on-disk data format, backward-compatible or not,
 //     forward-compatible or not, require updating the |kSimpleVersion|.
 //   * All cache Upgrades are performed on backend start, must be finished
 //     before the new backend starts processing any incoming operations.
 //   * If the Upgrade is not implemented for transition from
 //     |kSimpleVersion - 1| then the whole cache directory will be cleared.
 //   * Dropping cache data on disk or some of its parts can be a valid way to
 //     Upgrade.
-const uint32_t kSimpleVersion = 7;
+const uint32_t kLastCompatSparseVersion = 7;
+const uint32_t kSimpleVersion = 8;
 
 // The version of the entry file(s) as written to disk. Must be updated iff the
 // entry format changes with the overall backend version update.
 const uint32_t kSimpleEntryVersionOnDisk = 5;
 
 }  // namespace disk_cache
 
 #endif  // NET_DISK_CACHE_SIMPLE_SIMPLE_BACKEND_VERSION_H_