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

components/user_prefs/tracked/tracked_preferences_migration_unittest.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 "components/user_prefs/tracked/tracked_preferences_migration.h"
 
 #include <set>
 #include <string>
 #include <vector>
 
@@ skipping 242 matching lines @@
     DCHECK(store);
     const base::DictionaryValue* hash_store_contents =
         DictionaryHashStoreContents(store).GetContents();
     return hash_store_contents &&
            hash_store_contents->GetString(expected_pref_in_hash_store,
                                           static_cast<std::string*>(NULL));
   }
 
   // Determines whether |expected_pref_in_hash_store| has a hash in the Local
   // State hash store.
-  bool ContainsLegacyHash(std::string expected_pref_in_hash_store) {
+  bool ContainsLegacyHash(const std::string& expected_pref_in_hash_store) {
     const base::DictionaryValue* hash_store_contents =
         PrefServiceHashStoreContents(kHashStoreId, &local_state_).GetContents();
     return hash_store_contents &&
            hash_store_contents->GetString(expected_pref_in_hash_store,
                                           static_cast<std::string*>(NULL));
   }
 
   // Both stores need to hand their prefs over in order for migration to kick
   // in.
   void HandPrefsToMigrator(MockPrefStoreID store_id) {
@@ skipping 622 matching lines @@
                        expected_unprotected_values);
 
     base::StringPairs expected_protected_values;
     expected_protected_values.push_back(std::make_pair(
         kProtectedPref, kProtectedPrefValue));
     expected_protected_values.push_back(std::make_pair(
         kPreviouslyUnprotectedPref, kPreviouslyUnprotectedPrefValue));
     VerifyValuesStored(MOCK_PROTECTED_PREF_STORE, expected_protected_values);
   }
 }