Prevent popunders with the new auto-dismissing dialogs.

chrome/browser/ui/javascript_dialogs/javascript_dialog.cc

 // Copyright 2016 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/javascript_dialogs/javascript_dialog.h"
 
+#include "chrome/browser/ui/blocked_content/app_modal_dialog_helper.h"
 #include "chrome/browser/ui/javascript_dialogs/javascript_dialog_views.h"
+#include "content/public/browser/web_contents.h"
+#include "content/public/browser/web_contents_delegate.h"
+
+JavaScriptDialog::JavaScriptDialog(content::WebContents* parent_web_contents) {
+  dialog_helper_.reset(new AppModalDialogHelper(parent_web_contents));
+  parent_web_contents->GetDelegate()->ActivateContents(parent_web_contents);
+}
 
 JavaScriptDialog::~JavaScriptDialog() = default;
 
 base::WeakPtr<JavaScriptDialog> JavaScriptDialog::Create(
     content::WebContents* parent_web_contents,
     content::WebContents* alerting_web_contents,
     const base::string16& title,
     content::JavaScriptMessageType message_type,
     const base::string16& message_text,
     const base::string16& default_prompt_text,
     const content::JavaScriptDialogManager::DialogClosedCallback&
         dialog_callback) {
   return JavaScriptDialogViews::Create(
       parent_web_contents, alerting_web_contents, title, message_type,
       message_text, default_prompt_text, dialog_callback);
 }