Make RenderObject::style() return a const object

Source/core/css/resolver/ElementResolveContext.cpp

 /*
  * 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 31 matching lines @@
     : m_element(&element)
     , m_elementLinkState(element.document().visitedLinkState().determineLinkState(element))
     , m_distributedToInsertionPoint(false)
 {
     NodeRenderingTraversal::ParentDetails parentDetails;
     m_parentNode = NodeRenderingTraversal::parent(element, &parentDetails);
     m_distributedToInsertionPoint = parentDetails.insertionPoint();
 
     const Document& document = element.document();
     Node* documentElement = document.documentElement();
-    LayoutStyle* documentStyle = document.layoutStyle();
+    const LayoutStyle* documentStyle = document.layoutStyle();
     m_rootElementStyle = documentElement && element != documentElement ? documentElement->layoutStyle() : documentStyle;
     if (!m_rootElementStyle)
         m_rootElementStyle = documentStyle;
 }
 
 } // namespace blink