Change uses of base::JoinString to pass StringPieces where possible.

chrome/browser/sync_file_system/drive_backend/metadata_database.cc

 // 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.
 
 #include "chrome/browser/sync_file_system/drive_backend/metadata_database.h"
 
 #include <algorithm>
 #include <stack>
 #include <utility>
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/location.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/scoped_vector.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/single_thread_task_runner.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
+#include "base/strings/string_piece.h"
 #include "base/strings/string_util.h"
 #include "base/task_runner_util.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/threading/thread_task_runner_handle.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"
 #include "chrome/browser/sync_file_system/drive_backend/metadata_database_index.h"
 #include "chrome/browser/sync_file_system/drive_backend/metadata_database_index_interface.h"
@@ skipping 1706 matching lines @@
     dict->SetString("file_id", file_id);
     if (file.has_details()) {
       const FileDetails& details = file.details();
       dict->SetString("title", details.title());
       dict->SetString("type", FileKindToString(details.file_kind()));
       dict->SetString("md5", details.md5());
       dict->SetString("etag", details.etag());
       dict->SetString("missing", details.missing() ? "true" : "false");
       dict->SetString("change_id", base::Int64ToString(details.change_id()));
 
-      std::vector<std::string> parents;
+      std::vector<base::StringPiece> parents;
       for (int i = 0; i < details.parent_folder_ids_size(); ++i)
         parents.push_back(details.parent_folder_ids(i));
       dict->SetString("parents", base::JoinString(parents, ","));
     }
     files->Append(std::move(dict));
   }
   return files;
 }
 
 void MetadataDatabase::AttachSyncRoot(
@@ skipping 45 matching lines @@
       return false;
 
   if (!parents.empty())
     return false;
 
   return true;
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system