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

sync/internal_api/sync_encryption_handler_impl_unittest.cc

 // Copyright 2012 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 "sync/internal_api/sync_encryption_handler_impl.h"
 
 #include <string>
 
 #include "base/base64.h"
 #include "base/json/json_string_value_serializer.h"
@@ skipping 46 matching lines @@
                void(ModelTypeSet, bool));  // NOLINT
   MOCK_METHOD0(OnEncryptionComplete, void());  // NOLINT
   MOCK_METHOD1(OnCryptographerStateChanged, void(Cryptographer*));  // NOLINT
   MOCK_METHOD2(OnPassphraseTypeChanged, void(PassphraseType,
                                              base::Time));  // NOLINT
   MOCK_METHOD1(OnLocalSetPassphraseEncryption,
                void(const SyncEncryptionHandler::NigoriState&));  // NOLINT
 };
 
 google::protobuf::RepeatedPtrField<google::protobuf::string>
-BuildEncryptionKeyProto(std::string encryption_key) {
+BuildEncryptionKeyProto(const std::string& encryption_key) {
   google::protobuf::RepeatedPtrField<google::protobuf::string> keys;
   keys.Add()->assign(encryption_key);
   return keys;
 }
 
 }  // namespace
 
 class SyncEncryptionHandlerImplTest : public ::testing::Test {
  public:
   SyncEncryptionHandlerImplTest() {}
@@ skipping 2417 matching lines @@
   {
     ReadTransaction trans(FROM_HERE, user_share());
     encryption_handler()->SetKeystoreKeys(BuildEncryptionKeyProto(
                                               kRawKeystoreKey),
                                           trans.GetWrappedTrans());
 
   }
 }
 
 }  // namespace syncer