Avoid unchecked casts in UA shadow DOM.

third_party/WebKit/Source/core/html/HTMLKeygenElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2010 Apple Inc. All rights reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 109 matching lines @@
   DEFINE_STATIC_LOCAL(const AtomicString, keygen, ("keygen"));
   return keygen;
 }
 
 void HTMLKeygenElement::resetImpl() {
   shadowSelect()->reset();
 }
 
 HTMLSelectElement* HTMLKeygenElement::shadowSelect() const {
   ShadowRoot* root = userAgentShadowRoot();
-  return root ? toHTMLSelectElement(root->firstChild()) : 0;
+  return root ? toHTMLSelectElementOrDie(root->firstChild()) : nullptr;
 }
 
 bool HTMLKeygenElement::isInteractiveContent() const {
   return true;
 }
 
 bool HTMLKeygenElement::supportsAutofocus() const {
   return true;
 }
 
 }  // namespace blink