Fix a Windows crash bug with javascript alerts from extension popups.

chrome/browser/ui/app_modal_dialogs/app_modal_dialog.cc

 // Copyright (c) 2011 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/app_modal_dialogs/app_modal_dialog.h"
 
 #include "base/logging.h"
 #include "chrome/browser/ui/app_modal_dialogs/app_modal_dialog_queue.h"
 #include "chrome/browser/ui/app_modal_dialogs/native_app_modal_dialog.h"
 #include "chrome/common/chrome_notification_types.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/browser/web_contents_delegate.h"
 
 using content::WebContents;
 
 AppModalDialog::AppModalDialog(WebContents* web_contents, const string16& title)
     : valid_(true),
       native_dialog_(NULL),
       title_(title),
-      web_contents_(web_contents) {
+      web_contents_(web_contents),
+      completed_(false) {
 }
 
 AppModalDialog::~AppModalDialog() {
+  CompleteDialog();
 }
 
 void AppModalDialog::ShowModalDialog() {
   web_contents_->GetDelegate()->ActivateContents(web_contents_);
   CreateAndShowDialog();
 
   content::NotificationService::current()->Notify(
       chrome::NOTIFICATION_APP_MODAL_DIALOG_SHOWN,
       content::Source<AppModalDialog>(this),
       content::NotificationService::NoDetails());
@@ skipping 20 matching lines @@
   DCHECK(native_dialog_);
   native_dialog_->ActivateAppModalDialog();
 }
 
 void AppModalDialog::CloseModalDialog() {
   DCHECK(native_dialog_);
   native_dialog_->CloseAppModalDialog();
 }
 
 void AppModalDialog::CompleteDialog() {
-  AppModalDialogQueue::GetInstance()->ShowNextDialog();
+  if (!completed_) {
+    completed_ = true;
+    AppModalDialogQueue::GetInstance()->ShowNextDialog();
+  }
 }