Make RenderObject::style() return a const object

Source/core/html/HTMLOptionElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2004, 2005, 2006, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  * Copyright (C) 2011 Motorola Mobility, Inc.  All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 389 matching lines @@
 {
     // If m_style is not set, then the node is still unattached.
     // We have to wait till it gets attached to read the display property.
     if (!m_style)
         return false;
 
     if (m_style->display() != NONE) {
         Element* parent = parentElement();
         ASSERT(parent);
         if (isHTMLOptGroupElement(*parent)) {
-            LayoutStyle* parentStyle = parent->layoutStyle() ? parent->layoutStyle() : parent->computedStyle();
+            const LayoutStyle* parentStyle = parent->layoutStyle() ? parent->layoutStyle() : parent->computedStyle();
             return !parentStyle || parentStyle->display() == NONE;
         }
     }
     return m_style->display() == NONE;
 }
 
 } // namespace blink