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

chromeos/dbus/bluetooth_property.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_BLUETOOTH_PROPERTY_H_
 #define CHROMEOS_DBUS_BLUETOOTH_PROPERTY_H_
 #pragma once
 
 #include "base/compiler_specific.h"
 #include "dbus/message.h"
 #include "dbus/object_proxy.h"
 #include "dbus/property.h"
 #include "third_party/cros_system_api/dbus/service_constants.h"
 
 namespace chromeos {
 
 // BlueZ predates the common D-Bus Properties API (though it inspired it),
 // override dbus::PropertySet to generate the correct method call to get
 // all properties, conenect to the correct signal and parse it correctly.
 //
 // BluetoothPropertySet should be used with BluetoothProperty<>.
 class BluetoothPropertySet : public dbus::PropertySet {
  public:
   BluetoothPropertySet(dbus::ObjectProxy* object_proxy,
                        const std::string& interface,
-                       PropertyChangedCallback callback)
+                       const PropertyChangedCallback& callback)
       : dbus::PropertySet(object_proxy, interface, callback) {}
 
   // dbus::PropertySet override.
   //
   // Call after construction to connect property change notification
   // signals. Sub-classes may override to use different D-Bus signals.
   virtual void ConnectSignals() OVERRIDE;
 
   // dbus::PropertySet override.
   //
@@ skipping 50 matching lines @@
                              dbus::ObjectProxy::TIMEOUT_USE_DEFAULT,
                              base::Bind(&dbus::Property<T>::OnSet,
                                         this->GetWeakPtr(),
                                         callback));
   }
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_DBUS_BLUETOOTH_PROPERTY_H_