chromeos: Convert D-Bus client classes' callback arguments to const reference

chromeos/dbus/mock_image_burner_client.h

 // Copyright (c) 2012 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 CHROMEOS_DBUS_MOCK_IMAGE_BURNER_CLIENT_H_
 #define CHROMEOS_DBUS_MOCK_IMAGE_BURNER_CLIENT_H_
 #pragma once
 
 #include <string>
 
 #include "chromeos/dbus/image_burner_client.h"
 #include "testing/gmock/include/gmock/gmock.h"
 
 namespace chromeos {
 
 class MockImageBurnerClient : public ImageBurnerClient {
  public:
   MockImageBurnerClient();
   virtual ~MockImageBurnerClient();
 
-  MOCK_METHOD3(BurnImage,
-               void(const std::string&, const std::string&, ErrorCallback));
-  MOCK_METHOD2(SetEventHandlers,
-                void(BurnFinishedHandler, BurnProgressUpdateHandler));
+  MOCK_METHOD3(BurnImage, void(const std::string&,
+                               const std::string&,
+                               const ErrorCallback&));
+  MOCK_METHOD2(SetEventHandlers, void(const BurnFinishedHandler&,
+                                      const BurnProgressUpdateHandler&));
   MOCK_METHOD0(ResetEventHandlers, void());
 
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_DBUS_MOCK_IMAGE_BURNER_CLIENT_H_