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

chrome/browser/app_modal_dialog_win.cc

 // Copyright (c) 2009 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/app_modal_dialog.h"
 
 #include "base/logging.h"
 #include "chrome/browser/views/jsmessage_box_dialog.h"
 #include "views/window/dialog_delegate.h"
 #include "views/window/window.h"
 
 AppModalDialog::~AppModalDialog() {
 }
 
 void AppModalDialog::CreateAndShowDialog() {
   dialog_ = CreateNativeDialog();
   DCHECK(dialog_->IsModal());
   dialog_->ShowModalDialog();
 }
 
 void AppModalDialog::ActivateModalDialog() {
+  DCHECK(dialog_);
   dialog_->ActivateModalDialog();
 }
 
 void AppModalDialog::CloseModalDialog() {
+  DCHECK(dialog_);
   dialog_->CloseModalDialog();
 }