Make RenderObject::style() return a const object

Source/core/rendering/RenderListMarker.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Andrew Wellington (proton@wiretapped.net)
  * Copyright (C) 2010 Daniel Bates (dbates@intudata.com)
  *
  * 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 1445 matching lines @@
                         break;
                     default:
                         marginEnd = m_text.isEmpty() ? 0 : offset / 2;
                 }
             }
             marginStart = -marginEnd - minPreferredLogicalWidth();
         }
 
     }
 
-    style()->setMarginStart(Length(marginStart, Fixed));
-    style()->setMarginEnd(Length(marginEnd, Fixed));
+    mutableStyleRef().setMarginStart(Length(marginStart, Fixed));
+    mutableStyleRef().setMarginEnd(Length(marginEnd, Fixed));
 }
 
 LayoutUnit RenderListMarker::lineHeight(bool firstLine, LineDirectionMode direction, LinePositionMode linePositionMode) const
 {
     if (!isImage())
         return m_listItem->lineHeight(firstLine, direction, PositionOfInteriorLineBoxes);
     return RenderBox::lineHeight(firstLine, direction, linePositionMode);
 }
 
 int RenderListMarker::baselinePosition(FontBaseline baselineType, bool firstLine, LineDirectionMode direction, LinePositionMode linePositionMode) const
@@ skipping 161 matching lines @@
     if (style()) {
         // Reuse the current margins. Otherwise resetting the margins to initial values
         // would trigger unnecessary layout.
         newStyle->setMarginStart(style()->marginStart());
         newStyle->setMarginEnd(style()->marginRight());
     }
     setStyle(newStyle.release());
 }
 
 } // namespace blink