Fixed crash by replacing DCHECK with returning nullptr.

third_party/WebKit/Source/core/dom/NodeComputedStyle.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2008 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *
  * 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 34 matching lines @@
 {
     if (hasLayoutObject())
         return layoutObject()->mutableStyle();
     // <option> and <optgroup> can be styled even if they don't get layout objects,
     // so they store their style internally and return it through nonLayoutObjectComputedStyle().
     // We check here explicitly to avoid the virtual call in the common case.
     if (isHTMLOptGroupElement(*this) || isHTMLOptionElement(this))
         return nonLayoutObjectComputedStyle();
     if (hasRareData()) {
         NodeRareData* rareData = this->rareData();
-        DCHECK(rareData->isElementRareData());
+        if (!rareData->isElementRareData())
+            return nullptr;
         return static_cast<ElementRareData*>(rareData)->computedStyle();
     }
     return m_data.m_computedStyle;
 }
 
 inline const ComputedStyle* Node::parentComputedStyle() const
 {
     if (isSlotOrActiveInsertionPoint())
         return 0;
     ContainerNode* parent = LayoutTreeBuilderTraversal::parent(*this);
     return parent ? parent->computedStyle() : 0;
 }
 
 inline const ComputedStyle& Node::computedStyleRef() const
 {
     const ComputedStyle* style = computedStyle();
     DCHECK(style);
     return *style;
 }
 
 } // namespace blink
 #endif // NodeComputedStyle_h