Constify and use reference for RenderStyle in LayoutTheme

Source/core/rendering/RenderObject.h

Index: Source/core/rendering/RenderObject.h
diff --git a/Source/core/rendering/RenderObject.h b/Source/core/rendering/RenderObject.h
index 2338e8266e8a72c059078893967acebacacb394d..7580e666fb24379c93f0011d362f98e6db0d18c1 100644
--- a/Source/core/rendering/RenderObject.h
+++ b/Source/core/rendering/RenderObject.h
@@ -799,7 +799,8 @@ public:
     // 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;