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

chromeos/network/onc/onc_certificate_importer_impl_unittest.cc

Index: chromeos/network/onc/onc_certificate_importer_impl_unittest.cc
diff --git a/chromeos/network/onc/onc_certificate_importer_impl_unittest.cc b/chromeos/network/onc/onc_certificate_importer_impl_unittest.cc
index 35ff424149eec34d2566c132de663842e0f61632..f98cb5bdf5d09b3f51c4e57d39c8259e3af351d6 100644
--- a/chromeos/network/onc/onc_certificate_importer_impl_unittest.cc
+++ b/chromeos/network/onc/onc_certificate_importer_impl_unittest.cc
@@ -97,7 +97,8 @@ class ONCCertificateImporterImplTest : public testing::Test {
     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;