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

chromeos/network/onc/onc_certificate_importer_impl_unittest.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 "chromeos/network/onc/onc_certificate_importer_impl.h"
 
 #include <cert.h>
 #include <certdb.h>
 #include <keyhi.h>
 #include <pk11pub.h>
@@ skipping 79 matching lines @@
   }
 
  protected:
   void OnImportCompleted(bool expected_success,
                          bool success,
                          const net::CertificateList& onc_trusted_certificates) {
     EXPECT_EQ(expected_success, success);
     web_trust_certificates_ = onc_trusted_certificates;
   }
 
-  void AddCertificatesFromFile(std::string filename, bool expected_success) {
+  void AddCertificatesFromFile(const std::string& filename,
+                               bool expected_success) {
     scoped_ptr<base::DictionaryValue> onc =
         test_utils::ReadTestDictionary(filename);
     scoped_ptr<base::Value> certificates_value;
     base::ListValue* certificates = NULL;
     onc->RemoveWithoutPathExpansion(::onc::toplevel_config::kCertificates,
                                     &certificates_value);
     certificates_value.release()->GetAsList(&certificates);
     onc_certificates_.reset(certificates);
 
     web_trust_certificates_.clear();
@@ skipping 230 matching lines @@
                   "certificate-client-update.onc"),
         CertParam(net::SERVER_CERT,
                   "certificate-server.onc",
                   "certificate-server-update.onc"),
         CertParam(net::CA_CERT,
                   "certificate-web-authority.onc",
                   "certificate-web-authority-update.onc")));
 
 }  // namespace onc
 }  // namespace chromeos