Make RenderObject::style() return a const object

Source/core/css/resolver/StyleResolverState.h

Index: Source/core/css/resolver/StyleResolverState.h
diff --git a/Source/core/css/resolver/StyleResolverState.h b/Source/core/css/resolver/StyleResolverState.h
index 148696222720d12c6d7001849e9083406c1b88bc..561faa09011e16f9a2b91286ddeb7f6e4c7caa4f 100644
--- a/Source/core/css/resolver/StyleResolverState.h
+++ b/Source/core/css/resolver/StyleResolverState.h
@@ -45,8 +45,8 @@ class StyleResolverState {
     STACK_ALLOCATED();
     WTF_MAKE_NONCOPYABLE(StyleResolverState);
 public:
-    StyleResolverState(Document&, const ElementResolveContext&, RenderStyle* parentStyle);
-    StyleResolverState(Document&, Element*, RenderStyle* parentStyle = 0);
+    StyleResolverState(Document&, const ElementResolveContext&, const RenderStyle* parentStyle);
+    StyleResolverState(Document&, Element*, const RenderStyle* parentStyle = 0);
     ~StyleResolverState();
 
     // In FontFaceSet and CanvasRenderingContext2D, we don't have an element to grab the document from.
@@ -81,9 +81,13 @@ public:
     const CSSAnimationUpdate* animationUpdate() { return m_animationUpdate.get(); }
     PassOwnPtrWillBeRawPtr<CSSAnimationUpdate> takeAnimationUpdate();
 
-    void setParentStyle(PassRefPtr<RenderStyle> parentStyle) { m_parentStyle = parentStyle; }
-    const RenderStyle* parentStyle() const { return m_parentStyle.get(); }
-    RenderStyle* parentStyle() { return m_parentStyle.get(); }
+    void setParentStyle(PassRefPtr<RenderStyle> parentStyle) { m_overrideParentStyle = parentStyle; }
+    const RenderStyle* parentStyle() const
+    {
+        if (m_overrideParentStyle)
+            return m_overrideParentStyle.get();
+        return m_parentStyle;
+    }
 
     // FIXME: These are effectively side-channel "out parameters" for the various
     // map functions. When we map from CSS to style objects we use this state object
@@ -143,7 +147,8 @@ private:
 
     // m_parentStyle is not always just ElementResolveContext::parentStyle,
     // so we keep it separate.
-    RefPtr<RenderStyle> m_parentStyle;
+    const RenderStyle* m_parentStyle;
+    RefPtr<RenderStyle> m_overrideParentStyle;
 
     OwnPtrWillBeMember<CSSAnimationUpdate> m_animationUpdate;