Constify and use reference for RenderStyle in LayoutTheme

Source/core/rendering/RenderObject.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2004 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google 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
@@ skipping 781 matching lines @@
     LayoutRect paintingRootRect(LayoutRect& topLevelRect);
 
     virtual LayoutUnit minPreferredLogicalWidth() const { return 0; }
     virtual LayoutUnit maxPreferredLogicalWidth() const { return 0; }
 
     RenderStyle* style() const { return m_style.get(); }
 
     // m_style can only be nullptr before the first style is set, thus most
     // callers will never see a nullptr style and should use styleRef().
     // FIXME: It would be better if style() returned a const reference.
-    const RenderStyle& styleRef() const { ASSERT(m_style); return *m_style; }
+    const RenderStyle& styleRef() const { return mutableStyleRef(); }
+    RenderStyle& mutableStyleRef() const { ASSERT(m_style); return *m_style; }
 
     /* The following methods are inlined in RenderObjectInlines.h */
     RenderStyle* firstLineStyle() const;
     RenderStyle* style(bool firstLine) const;
     const RenderStyle& styleRef(bool firstLine) const;
 
     static inline Color resolveColor(const RenderStyle& styleToUse, int colorProperty)
     {
         return styleToUse.visitedDependentColor(colorProperty);
     }
@@ skipping 799 matching lines @@
 void showTree(const blink::RenderObject*);
 void showLineTree(const blink::RenderObject*);
 void showRenderTree(const blink::RenderObject* object1);
 // We don't make object2 an optional parameter so that showRenderTree
 // can be called from gdb easily.
 void showRenderTree(const blink::RenderObject* object1, const blink::RenderObject* object2);
 
 #endif
 
 #endif // RenderObject_h