GTK: More raw struct access removal, this time focusing on GtkDialog.

chrome/browser/ui/gtk/html_dialog_gtk.cc

Index: chrome/browser/ui/gtk/html_dialog_gtk.cc
diff --git a/chrome/browser/ui/gtk/html_dialog_gtk.cc b/chrome/browser/ui/gtk/html_dialog_gtk.cc
index 6a03b92902ed22fa8bebb356487713908305867c..8c3d33c4f1cca1ba7b4c71e15334ee4ae344ee6b 100644
--- a/chrome/browser/ui/gtk/html_dialog_gtk.cc
+++ b/chrome/browser/ui/gtk/html_dialog_gtk.cc
@@ -203,7 +203,8 @@ gfx::NativeWindow HtmlDialogGtk::InitDialog() {
   g_signal_connect(dialog_, "response", G_CALLBACK(OnResponseThunk), this);
 
   tab_contents_container_.reset(new TabContentsContainerGtk(NULL));
-  gtk_box_pack_start(GTK_BOX(GTK_DIALOG(dialog_)->vbox),
+  GtkWidget* content_area = gtk_dialog_get_content_area(GTK_DIALOG(dialog_));
+  gtk_box_pack_start(GTK_BOX(content_area),
                      tab_contents_container_->widget(), TRUE, TRUE, 0);
 
   tab_contents_container_->SetTab(tab_.get());