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

chromeos/dbus/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_IMAGE_BURNER_CLIENT_H_
 #define CHROMEOS_DBUS_IMAGE_BURNER_CLIENT_H_
 #pragma once
 
 #include <string>
 
@@ skipping 24 matching lines @@
                               const std::string& error)> BurnFinishedHandler;
 
   // A callback to handle burn_progress_update signal.
   typedef base::Callback<void(const std::string& target_path,
                               int64 num_bytes_burnt,
                               int64 total_size)> BurnProgressUpdateHandler;
 
   // Burns the image |from_path| to the disk |to_path|.
   virtual void BurnImage(const std::string& from_path,
                          const std::string& to_path,
-                         ErrorCallback error_callback) = 0;
+                         const ErrorCallback& error_callback) = 0;
 
   // Sets callbacks as event handlers.
   // |burn_finished_handler| is called when burn_finished signal is received.
   // |burn_progress_update_handler| is called when burn_progress_update signal
   // is received.
   virtual void SetEventHandlers(
-      BurnFinishedHandler burn_finished_handler,
-      BurnProgressUpdateHandler burn_progress_update_handler) = 0;
+      const BurnFinishedHandler& burn_finished_handler,
+      const BurnProgressUpdateHandler& burn_progress_update_handler) = 0;
 
   // Resets event handlers. After calling this method, nothing is done when
   // signals are received.
   virtual void ResetEventHandlers() = 0;
 
   // Factory function, creates a new instance and returns ownership.
   // For normal usage, access the singleton via DBusThreadManager::Get().
   static ImageBurnerClient* Create(DBusClientImplementationType type,
                                    dbus::Bus* bus);
 
  protected:
   // Create() should be used instead.
   ImageBurnerClient();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ImageBurnerClient);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_DBUS_IMAGE_BURNER_CLIENT_H_