Pass u2f message callback vector as a const reference

device/u2f/u2f_request.h

 // 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.
 
 #ifndef DEVICE_U2F_U2F_REQUEST_H_
 #define DEVICE_U2F_U2F_REQUEST_H_
 
 #include "base/cancelable_callback.h"
 #include "base/scoped_observer.h"
 #include "device/hid/hid_device_filter.h"
 #include "device/hid/hid_service.h"
 #include "u2f_device.h"
 
 namespace device {
 class U2fRequest : HidService::Observer {
  public:
-  using ResponseCallback = base::Callback<void(U2fReturnCode status_code,
-                                               std::vector<uint8_t> response)>;
+  using ResponseCallback =
+      base::Callback<void(U2fReturnCode status_code,
+                          const std::vector<uint8_t>& response)>;
 
   U2fRequest(const ResponseCallback& callback);
   virtual ~U2fRequest();
 
   void Start();
   void AddDeviceForTesting(std::unique_ptr<U2fDevice> device);
 
  protected:
   enum class State {
     INIT,
@@ skipping 27 matching lines @@
   std::list<std::unique_ptr<U2fDevice>> devices_;
   std::list<std::unique_ptr<U2fDevice>> attempted_devices_;
   base::CancelableClosure delay_callback_;
   HidDeviceFilter filter_;
   ScopedObserver<HidService, HidService::Observer> hid_service_observer_;
   base::WeakPtrFactory<U2fRequest> weak_factory_;
 };
 }  // namespace device
 
 #endif  // DEVICE_U2F_U2F_REQUEST_H_