Pass u2f message callback vector as a const reference

device/u2f/u2f_register_unittest.cc

 // Copyright 2017 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 <list>
 
 #include "base/run_loop.h"
 #include "base/test/test_io_thread.h"
 #include "device/base/mock_device_client.h"
 #include "device/hid/mock_hid_service.h"
@@ skipping 20 matching lines @@
 };
 
 class TestRegisterCallback {
  public:
   TestRegisterCallback()
       : callback_(base::Bind(&TestRegisterCallback::ReceivedCallback,
                              base::Unretained(this))) {}
   ~TestRegisterCallback() {}
 
   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();
   }
 
   std::pair<U2fReturnCode, std::vector<uint8_t>>& WaitForCallback() {
     closure_ = run_loop_.QuitClosure();
     run_loop_.Run();
     return response_;
   }
 
@@ skipping 69 matching lines @@
   request->AddDeviceForTesting(std::move(device0));
   request->AddDeviceForTesting(std::move(device1));
   std::pair<U2fReturnCode, std::vector<uint8_t>>& response =
       cb.WaitForCallback();
   EXPECT_EQ(U2fReturnCode::SUCCESS, response.first);
   ASSERT_LT(static_cast<size_t>(0), response.second.size());
   EXPECT_EQ(static_cast<uint8_t>(MockU2fDevice::kRegister), response.second[0]);
 }
 
 }  // namespace device