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

chromeos/dbus/power_manager_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_POWER_MANAGER_CLIENT_H_
 #define CHROMEOS_DBUS_POWER_MANAGER_CLIENT_H_
 
 #include <string>
 
 #include "base/basictypes.h"
@@ skipping 144 matching lines @@
   // the request id and the method will call the provided callback with the
   // new request ID for use with further calls.
   // The overrides parameter will & out the PowerStateOverrideType types to
   // allow specific selection of overrides. For example, to override just dim
   // and suspending but leaving blanking in, set overrides to,
   // DISABLE_IDLE_DIM | DISABLE_IDLE_SUSPEND.
   virtual void RequestPowerStateOverrides(
       uint32 request_id,
       uint32 duration,
       int overrides,
-      PowerStateRequestIdCallback callback) = 0;
+      const PowerStateRequestIdCallback& callback) = 0;
 
   // Creates the instance.
   static PowerManagerClient* Create(DBusClientImplementationType type,
                                     dbus::Bus* bus);
 
   virtual ~PowerManagerClient();
 
  protected:
   // Create() should be used instead.
   PowerManagerClient();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(PowerManagerClient);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_DBUS_POWER_MANAGER_CLIENT_H_