Pass u2f message callback vector as a const reference

device/u2f/u2f_sign_unittest.cc

Index: device/u2f/u2f_sign_unittest.cc
diff --git a/device/u2f/u2f_sign_unittest.cc b/device/u2f/u2f_sign_unittest.cc
index 8c846217ae9ca9f8ff900dc42729cace87905009..95650cad6885b9400223ed4ca5f7289ee1c27e1d 100644
--- a/device/u2f/u2f_sign_unittest.cc
+++ b/device/u2f/u2f_sign_unittest.cc
@@ -37,7 +37,7 @@ class TestSignCallback {
   ~TestSignCallback() {}
 
   void ReceivedCallback(U2fReturnCode status_code,
-                        std::vector<uint8_t> response) {
+                        const std::vector<uint8_t>& response) {
     response_ = std::make_pair(status_code, response);
     closure_.Run();
   }