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

chrome/utility/importer/firefox_importer_unittest_utils_mac.cc

Index: chrome/utility/importer/firefox_importer_unittest_utils_mac.cc
diff --git a/chrome/utility/importer/firefox_importer_unittest_utils_mac.cc b/chrome/utility/importer/firefox_importer_unittest_utils_mac.cc
index 28568499d91430e9bef273fa8994adba668965d2..a1b1c0435f0c14afd33893a8b4a7caee7b3ab35c 100644
--- a/chrome/utility/importer/firefox_importer_unittest_utils_mac.cc
+++ b/chrome/utility/importer/firefox_importer_unittest_utils_mac.cc
@@ -245,7 +245,7 @@ class FFDecryptorClientChannelListener : public IPC::Listener {
     sender_->Send(new Msg_Decryptor_InitReturnCode(ret));
   }
 
-  void OnDecrypt(std::string crypt) {
+  void OnDecrypt(const std::string& crypt) {
     base::string16 unencrypted_str = decryptor_.Decrypt(crypt);
     sender_->Send(new Msg_Decryptor_Response(unencrypted_str));
   }