fix: dialog DCHECK on Linux (#28532)

Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>

shell/browser/ui/file_dialog_gtk.cc

@@ -75,6 +75,7 @@ class FileChooserDialog {
       gtk_file_chooser_set_create_folders(GTK_FILE_CHOOSER(dialog_), TRUE);
 
     if (!settings.default_path.empty()) {
+      base::ThreadRestrictions::ScopedAllowIO allow_io;
       if (base::DirectoryExists(settings.default_path)) {
         gtk_file_chooser_set_current_folder(
             GTK_FILE_CHOOSER(dialog_), settings.default_path.value().c_str());