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

net/disk_cache/simple/simple_backend_version.h

Index: net/disk_cache/simple/simple_backend_version.h
diff --git a/net/disk_cache/simple/simple_backend_version.h b/net/disk_cache/simple/simple_backend_version.h
index 604d0e1a5962b3b82e0c7e97fe3ea9d681b5e1a6..e7a0ce6b166db16b6a1f4587d367e6179c90dea0 100644
--- a/net/disk_cache/simple/simple_backend_version.h
+++ b/net/disk_cache/simple/simple_backend_version.h
@@ -18,7 +18,8 @@ namespace disk_cache {
 //     |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.