Gtk3 ui: Add libgtk3ui as a separate build component

chrome/browser/ui/libgtk2ui/chrome_gtk_menu_subclasses.cc

-// Copyright 2013 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/chrome_gtk_menu_subclasses.h"
-
-G_DEFINE_TYPE(GtkCustomMenu, gtk_custom_menu, GTK_TYPE_MENU)
-
-static void gtk_custom_menu_init(GtkCustomMenu* menu) {
-}
-
-static void gtk_custom_menu_class_init(GtkCustomMenuClass* klass) {
-}
-
-GtkWidget* gtk_custom_menu_new() {
-  return GTK_WIDGET(g_object_new(gtk_custom_menu_get_type(), NULL));
-}
-
-G_DEFINE_TYPE(GtkCustomMenuItem, gtk_custom_menu_item, GTK_TYPE_MENU_ITEM)
-
-static void gtk_custom_menu_item_init(GtkCustomMenuItem* item) {
-}
-
-static void gtk_custom_menu_item_class_init(GtkCustomMenuItemClass* klass) {
-}
-
-GtkWidget* gtk_custom_menu_item_new() {
-  return GTK_WIDGET(g_object_new(gtk_custom_menu_item_get_type(), NULL));
-}