Move FileEnumerator to its own file, do some refactoring.

net/disk_cache/simple/simple_index.cc

 // 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.
 
 #include "net/disk_cache/simple/simple_index.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/file_util.h"
+#include "base/files/file_enumerator.h"
 #include "base/logging.h"
 #include "base/message_loop.h"
 #include "base/metrics/histogram.h"
 #include "base/pickle.h"
 #include "base/task_runner.h"
 #include "base/threading/worker_pool.h"
 #include "base/time.h"
 #include "net/base/net_errors.h"
 #include "net/disk_cache/simple/simple_entry_format.h"
 #include "net/disk_cache/simple/simple_index_file.h"
@@ skipping 378 matching lines @@
                                  start - last_write_to_disk_);
     }
   }
   last_write_to_disk_ = start;
 
   index_file_->WriteToDisk(entries_set_, cache_size_,
                            start, app_on_background_);
 }
 
 }  // namespace disk_cache