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

chromeos/dbus/session_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_SESSION_MANAGER_CLIENT_H_
 #define CHROMEOS_DBUS_SESSION_MANAGER_CLIENT_H_
 
 #include "base/callback.h"
 #include "base/observer_list.h"
 #include "chromeos/chromeos_export.h"
@@ skipping 42 matching lines @@
   // Stops the current session.
   virtual void StopSession() = 0;
 
   // Used for RetrieveDevicePolicy and RetrieveUserPolicy. Takes a serialized
   // protocol buffer as string.  Upon success, we will pass a protobuf to the
   // callback.  On failure, we will pass "".
   typedef base::Callback<void(const std::string&)> RetrievePolicyCallback;
 
   // Fetches the device policy blob stored by the session manager.  Upon
   // completion of the retrieve attempt, we will call the provided callback.
-  virtual void RetrieveDevicePolicy(RetrievePolicyCallback callback) = 0;
+  virtual void RetrieveDevicePolicy(const RetrievePolicyCallback& callback) = 0;
 
   // Fetches the user policy blob stored by the session manager for the
   // currently signed-in user.  Upon completion of the retrieve attempt, we will
   // call the provided callback.
-  virtual void RetrieveUserPolicy(RetrievePolicyCallback callback) = 0;
+  virtual void RetrieveUserPolicy(const RetrievePolicyCallback& callback) = 0;
 
   // Used for StoreDevicePolicy and StoreUserPolicy. Takes a boolean indicating
   // whether the operation was successful or not.
   typedef base::Callback<void(bool)> StorePolicyCallback;
 
   // Attempts to asynchronously store |policy_blob| as device policy.  Upon
   // completion of the store attempt, we will call callback.
   virtual void StoreDevicePolicy(const std::string& policy_blob,
-                                 StorePolicyCallback callback) = 0;
+                                 const StorePolicyCallback& callback) = 0;
 
   // Attempts to asynchronously store |policy_blob| as user policy for the
   // currently signed-in user.  Upon completion of the store attempt, we will
   // call callback.
   virtual void StoreUserPolicy(const std::string& policy_blob,
-                               StorePolicyCallback callback) = 0;
+                               const StorePolicyCallback& callback) = 0;
 
   // Creates the instance.
   static SessionManagerClient* Create(DBusClientImplementationType type,
                                       dbus::Bus* bus);
 
   virtual ~SessionManagerClient();
 
  protected:
   // Create() should be used instead.
   SessionManagerClient();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SessionManagerClient);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_DBUS_SESSION_MANAGER_CLIENT_H_