Fix -Wsential warnings

chrome/browser/ui/libgtk2ui/select_file_dialog_impl_gtk2.cc

 // 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.
 
 #include "chrome/browser/ui/libgtk2ui/select_file_dialog_impl_gtk2.h"
 
 #include <gtk/gtk.h>
 #include <stddef.h>
 #include <sys/stat.h>
 #include <sys/types.h>
@@ skipping 258 matching lines @@
 
 GtkWidget* SelectFileDialogImplGTK::CreateFileOpenHelper(
     const std::string& title,
     const base::FilePath& default_path,
     gfx::NativeWindow parent) {
   GtkWidget* dialog =
       gtk_file_chooser_dialog_new(title.c_str(), NULL,
                                   GTK_FILE_CHOOSER_ACTION_OPEN,
                                   "_Cancel", GTK_RESPONSE_CANCEL,
                                   "_Open", GTK_RESPONSE_ACCEPT,
-                                  NULL);
+                                  nullptr);
   SetGtkTransientForAura(dialog, parent);
   AddFilters(GTK_FILE_CHOOSER(dialog));
 
   if (!default_path.empty()) {
     if (CallDirectoryExistsOnUIThread(default_path)) {
       gtk_file_chooser_set_current_folder(GTK_FILE_CHOOSER(dialog),
                                           default_path.value().c_str());
     } else {
       // If the file doesn't exist, this will just switch to the correct
       // directory. That's good enough.
@@ skipping 21 matching lines @@
   std::string accept_button_label = (type == SELECT_UPLOAD_FOLDER) ?
       l10n_util::GetStringUTF8(IDS_SELECT_UPLOAD_FOLDER_DIALOG_UPLOAD_BUTTON) :
       "_Open";
 
   GtkWidget* dialog =
       gtk_file_chooser_dialog_new(title_string.c_str(), NULL,
                                   GTK_FILE_CHOOSER_ACTION_SELECT_FOLDER,
                                   "_Cancel", GTK_RESPONSE_CANCEL,
                                   accept_button_label.c_str(),
                                   GTK_RESPONSE_ACCEPT,
-                                  NULL);
+                                  nullptr);
   SetGtkTransientForAura(dialog, parent);
 
   if (!default_path.empty()) {
     gtk_file_chooser_set_filename(GTK_FILE_CHOOSER(dialog),
                                   default_path.value().c_str());
   } else if (!last_opened_path_->empty()) {
     gtk_file_chooser_set_current_folder(GTK_FILE_CHOOSER(dialog),
                                         last_opened_path_->value().c_str());
   }
   gtk_file_chooser_set_select_multiple(GTK_FILE_CHOOSER(dialog), FALSE);
@@ skipping 33 matching lines @@
 GtkWidget* SelectFileDialogImplGTK::CreateSaveAsDialog(const std::string& title,
     const base::FilePath& default_path, gfx::NativeWindow parent) {
   std::string title_string = !title.empty() ? title :
         l10n_util::GetStringUTF8(IDS_SAVE_AS_DIALOG_TITLE);
 
   GtkWidget* dialog =
       gtk_file_chooser_dialog_new(title_string.c_str(), NULL,
                                   GTK_FILE_CHOOSER_ACTION_SAVE,
                                   "_Cancel", GTK_RESPONSE_CANCEL,
                                   "_Save", GTK_RESPONSE_ACCEPT,
-                                  NULL);
+                                  nullptr);
   SetGtkTransientForAura(dialog, parent);
 
   AddFilters(GTK_FILE_CHOOSER(dialog));
   if (!default_path.empty()) {
     if (CallDirectoryExistsOnUIThread(default_path)) {
       // If this is an existing directory, navigate to that directory, with no
       // filename.
       gtk_file_chooser_set_current_folder(GTK_FILE_CHOOSER(dialog),
                                           default_path.value().c_str());
     } else {
@@ skipping 145 matching lines @@
   g_free(filename);
   if (pixbuf) {
     gtk_image_set_from_pixbuf(GTK_IMAGE(preview_), pixbuf);
     g_object_unref(pixbuf);
   }
   gtk_file_chooser_set_preview_widget_active(GTK_FILE_CHOOSER(chooser),
                                              pixbuf ? TRUE : FALSE);
 }
 
 }  // namespace libgtk2ui