Fix a crash when we try to close a js dialog that wasn't shown.

chrome/browser/app_modal_dialog_gtk.cc

Index: chrome/browser/app_modal_dialog_gtk.cc
diff --git a/chrome/browser/app_modal_dialog_gtk.cc b/chrome/browser/app_modal_dialog_gtk.cc
index b0268e6b6be0bfb0d5e46ed16bea56ae1ca628e7..2eb91854474dc5740ccb2504a822365b614ee08e 100644
--- a/chrome/browser/app_modal_dialog_gtk.cc
+++ b/chrome/browser/app_modal_dialog_gtk.cc
@@ -31,17 +31,21 @@ void AppModalDialog::OnDialogResponse(GtkDialog* dialog, gint response_id,
 }
 
 void AppModalDialog::ActivateModalDialog() {
+  DCHECK(dialog_);
   gtk_window_present(GTK_WINDOW(dialog_));
 }
 
 void AppModalDialog::CloseModalDialog() {
+  DCHECK(dialog_);
   HandleDialogResponse(GTK_DIALOG(dialog_), GTK_RESPONSE_DELETE_EVENT);
 }
 
 void AppModalDialog::AcceptWindow() {
+  DCHECK(dialog_);
   HandleDialogResponse(GTK_DIALOG(dialog_), GTK_RESPONSE_OK);
 }
 
 void AppModalDialog::CancelWindow() {
+  DCHECK(dialog_);
   HandleDialogResponse(GTK_DIALOG(dialog_), GTK_RESPONSE_CANCEL);
 }