Make RenderObject::style() return a const object

Source/core/css/resolver/ElementResolveContext.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 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
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 25 matching lines @@
 class ElementResolveContext {
     STACK_ALLOCATED();
 public:
     explicit ElementResolveContext(const Document&);
 
     explicit ElementResolveContext(Element&);
 
     Element* element() const { return m_element; }
     const ContainerNode* parentNode() const { return m_parentNode; }
     const LayoutStyle* rootElementStyle() const { return m_rootElementStyle; }
-    LayoutStyle* parentStyle() const { return parentNode() ? parentNode()->layoutStyle() : nullptr; }
+    const LayoutStyle* parentStyle() const { return parentNode() ? parentNode()->layoutStyle() : nullptr; }
     EInsideLink elementLinkState() const { return m_elementLinkState; }
     bool distributedToInsertionPoint() const { return m_distributedToInsertionPoint; }
 
 private:
     RawPtrWillBeMember<Element> m_element;
     RawPtrWillBeMember<ContainerNode> m_parentNode;
-    LayoutStyle* m_rootElementStyle;
+    const LayoutStyle* m_rootElementStyle;
     EInsideLink m_elementLinkState;
     bool m_distributedToInsertionPoint;
 };
 
 } // namespace blink
 
 #endif // ElementResolveContext_h