dbus: Reuse is_set() function from ScopedDBusError API.

dbus/scoped_dbus_error.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 DBUS_SCOPED_DBUS_ERROR_H_
 #define DBUS_SCOPED_DBUS_ERROR_H_
 
 #include <dbus/dbus.h>
 
 namespace dbus {
 
 // Utility class to ensure that DBusError is freed.
 class ScopedDBusError {
  public:
   ScopedDBusError() {
     dbus_error_init(&error_);
   }
 
   ~ScopedDBusError() {
     dbus_error_free(&error_);
   }
 
   DBusError* get() { return &error_; }
-  bool is_set() { return dbus_error_is_set(&error_); }
+  bool is_set() const { return dbus_error_is_set(&error_); }
   const char* name() { return error_.name; }
   const char* message() { return error_.message; }
 
  private:
   DBusError error_;
 };
 
 }  // namespace dbus
 
 #endif  // DBUS_SCOPED_DBUS_ERROR_H_