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

chromeos/dbus/cashew_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_CASHEW_CLIENT_H_
 #define CHROMEOS_DBUS_CASHEW_CLIENT_H_
 #pragma once
 
 #include <string>
 #include <vector>
@@ skipping 26 matching lines @@
       DataPlansUpdateHandler;
 
   virtual ~CashewClient();
 
   // Factory function, creates a new instance and returns ownership.
   // For normal usage, access the singleton via DBusThreadManager::Get().
   static CashewClient* Create(DBusClientImplementationType type,
                               dbus::Bus* bus);
 
   // Sets DataPlansUpdate signal handler.
-  virtual void SetDataPlansUpdateHandler(DataPlansUpdateHandler handler) = 0;
+  virtual void SetDataPlansUpdateHandler(
+      const DataPlansUpdateHandler& handler) = 0;
 
   // Resets DataPlansUpdate signal handler.
   virtual void ResetDataPlansUpdateHandler() = 0;
 
   // Calls RequestDataPlansUpdate method.
   virtual void RequestDataPlansUpdate() = 0;
 
  protected:
   // Create() should be used instead.
   CashewClient();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(CashewClient);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_DBUS_CASHEW_CLIENT_H_