Cleanup: Pass std::string as const reference if possible

chrome/browser/android/history_report/delta_file_backend_leveldb.cc

 // Copyright 2015 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/android/history_report/delta_file_backend_leveldb.h"
 
 #include <inttypes.h>
 #include "base/files/file_util.h"
 #include "base/logging.h"
 #include "base/strings/string_number_conversions.h"
@@ skipping 19 matching lines @@
   int64 seq_no = 0;
   if (db_iter->Valid()) {
     history_report::DeltaFileEntry last_entry;
     leveldb::Slice value_slice = db_iter->value();
     if (last_entry.ParseFromArray(value_slice.data(), value_slice.size()))
       seq_no = last_entry.seq_no();
   }
   return seq_no;
 }
 
-void SaveChange(leveldb::DB* db, std::string url, std::string type) {
+void SaveChange(leveldb::DB* db,
+                const std::string& url,
+                const std::string& type) {
   int64 seq_no = GetLastSeqNo(db) + 1;
   history_report::DeltaFileEntry entry;
   entry.set_seq_no(seq_no);
   entry.set_type(type);
   entry.set_url(url);
   leveldb::WriteOptions writeOptions;
   std::string key;
   base::SStringPrintf(&key, "%" PRId64, seq_no);
   leveldb::Status status = db->Put(
       writeOptions,
@@ skipping 177 matching lines @@
   leveldb::ReadOptions options;
   scoped_ptr<leveldb::Iterator> db_it(db_->NewIterator(options));
   int num_entries = 0;
   for (db_it->SeekToFirst(); db_it->Valid(); db_it->Next()) num_entries++;
   dump.append(base::IntToString(num_entries));
   dump.append("]");
   return dump;
 }
 
 }  // namespace history_report