Replace uses of hash_map in //base

chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.cc

 // Copyright 2014 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 "chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.h"
 
+#include <unordered_set>
+
 #include "base/format_macros.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "chrome/browser/sync_file_system/drive_backend/drive_backend_constants.h"
 #include "chrome/browser/sync_file_system/drive_backend/drive_backend_util.h"
 #include "chrome/browser/sync_file_system/drive_backend/leveldb_wrapper.h"
 #include "chrome/browser/sync_file_system/drive_backend/metadata_database.pb.h"
@@ skipping 185 matching lines @@
       }
 
       AppendContents(
           LookUpMap(trackers_by_parent, tracker_id, std::set<int64_t>()),
           &pending);
     }
   }
 
   // Delete all unreachable trackers, and list all |file_id| referred by
   // remained trackers.
-  base::hash_set<std::string> referred_file_ids;
+  std::unordered_set<std::string> referred_file_ids;
   {
     std::unique_ptr<LevelDBWrapper::Iterator> itr = db->NewIterator();
     for (itr->Seek(kFileTrackerKeyPrefix); itr->Valid(); itr->Next()) {
       if (!RemovePrefix(itr->key().ToString(), kFileTrackerKeyPrefix, nullptr))
         break;
 
       std::unique_ptr<FileTracker> tracker(new FileTracker);
       if (!tracker->ParseFromString(itr->value().ToString())) {
         util::Log(logging::LOG_WARNING, FROM_HERE,
                   "Failed to parse a Tracker");
@@ skipping 989 matching lines @@
   for (itr->Seek(prefix); itr->Valid();) {
     const std::string key = itr->key().ToString();
     if (!base::StartsWith(key, prefix, base::CompareCase::SENSITIVE))
       break;
     itr->Delete();
   }
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system