Avoid unchecked casts in UA shadow DOM.

third_party/WebKit/Source/core/svg/SVGUseElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Nikolas Zimmermann
  * <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007 Rob Buis <buis@kde.org>
  * Copyright (C) Research In Motion Limited 2009-2010. All rights reserved.
  * Copyright (C) 2011 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012 University of Szeged
  * Copyright (C) 2012 Renata Hodovan <reni@webkit.org>
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 433 matching lines @@
 
   // Expand all <use> elements in the shadow tree.
   // Expand means: replace the actual <use> element by what it references.
   if (!expandUseElementsInShadowTree()) {
     clearShadowTree();
     return;
   }
 
   // If the instance root was a <use>, it could have been replaced now, so
   // reset |m_targetElementInstance|.
-  m_targetElementInstance = toSVGElement(shadowTreeRootElement->firstChild());
+  m_targetElementInstance =
+      toSVGElementOrDie(shadowTreeRootElement->firstChild());
   ASSERT(m_targetElementInstance->parentNode() == shadowTreeRootElement);
 
   // Update relative length information.
   updateRelativeLengthsInformation();
 }
 
 LayoutObject* SVGUseElement::createLayoutObject(const ComputedStyle&) {
   return new LayoutSVGTransformableContainer(this);
 }
 
@@ skipping 264 matching lines @@
 
   if (m_resource)
     m_resource->removeClient(this);
 
   m_resource = resource;
   if (m_resource)
     m_resource->addClient(this);
 }
 
 }  // namespace blink