dbus: Reuse is_set() function from ScopedDBusError API.

dbus/scoped_dbus_error.h

Index: dbus/scoped_dbus_error.h
diff --git a/dbus/scoped_dbus_error.h b/dbus/scoped_dbus_error.h
index 67056477e5b3d6f7dcbdf7210dc0c5e3f257100b..c15c44397b91c59e21d871228b048f64c241ac55 100644
--- a/dbus/scoped_dbus_error.h
+++ b/dbus/scoped_dbus_error.h
@@ -21,7 +21,7 @@ class ScopedDBusError {
   }
 
   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; }