Make synchronous methods on disk_cache::SimpleEntryImpl() reentrant.

net/disk_cache/simple/simple_disk_format.h

 // Copyright (c) 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_DISK_FORMAT_H_
 #define NET_DISK_CACHE_SIMPLE_SIMPLE_DISK_FORMAT_H_
 
 namespace disk_cache {
 
 const uint64 kSimpleInitialMagicNumber = 0xfcfb6d1ba7725c30;
 
 // A file in the Simple cache consists of a SimpleFileHeader followed
 // by data.
 
 const uint32 kSimpleVersion = 1;
 
+static const int kSimpleEntryFileCount = 3;
+
 struct SimpleFileHeader {
   uint64 initial_magic_number;
   uint32 version;
   uint32 key_length;
   uint32 key_hash;
 };
 
 }  // namespace disk_cache
 
 #endif  // NET_DISK_CACHE_SIMPLE_SIMPLE_DISK_FORMAT_H_