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

chromeos/dbus/update_engine_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_UPDATE_ENGINE_CLIENT_H_
 #define CHROMEOS_DBUS_UPDATE_ENGINE_CLIENT_H_
 
 #include "base/callback.h"
 #include "base/observer_list.h"
 #include "chromeos/chromeos_export.h"
@@ skipping 64 matching lines @@
   virtual void AddObserver(Observer* observer) = 0;
   virtual void RemoveObserver(Observer* observer) = 0;
   // Returns true if this object has the given observer.
   virtual bool HasObserver(Observer* observer) = 0;
 
   // Called once RequestUpdateCheck() is complete. Takes one parameter:
   // - UpdateCheckResult: the result of the update check.
   typedef base::Callback<void(UpdateCheckResult)> UpdateCheckCallback;
 
   // Requests an update check and calls |callback| when completed.
-  virtual void RequestUpdateCheck(UpdateCheckCallback callback) = 0;
+  virtual void RequestUpdateCheck(const UpdateCheckCallback& callback) = 0;
 
   // Reboots if update has been performed.
   virtual void RebootAfterUpdate() = 0;
 
   // Requests to set the release track (channel). |track| should look like
   // "beta-channel" or "dev-channel".
   virtual void SetReleaseTrack(const std::string& track) = 0;
 
   // Called once GetReleaseTrack() is complete. Takes one parameter;
   // - string: the release track name like "beta-channel".
   typedef base::Callback<void(const std::string&)> GetReleaseTrackCallback;
 
   // Requests to get the release track and calls |callback| with the
   // release track (channel). On error, calls |callback| with an empty
   // string.
-  virtual void GetReleaseTrack(GetReleaseTrackCallback callback) = 0;
+  virtual void GetReleaseTrack(const GetReleaseTrackCallback& callback) = 0;
 
   // Returns the last status the object received from the update engine.
   //
   // Ideally, the D-Bus client should be state-less, but there are clients
   // that need this information.
   virtual Status GetLastStatus() = 0;
 
   // Returns an empty UpdateCheckCallback that does nothing.
   static UpdateCheckCallback EmptyUpdateCheckCallback();
 
   // Creates the instance.
   static UpdateEngineClient* Create(DBusClientImplementationType type,
                                     dbus::Bus* bus);
 
  protected:
   // Create() should be used instead.
   UpdateEngineClient();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(UpdateEngineClient);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_DBUS_UPDATE_ENGINE_CLIENT_H_