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 f10d3f6616eae3d972732a8c440dc1275e7a220b..a2ee2882b59e82a007e5719ce02e234a6ad956d4 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&, LayoutStyle* parentStyle);
-    StyleResolverState(Document&, Element*, LayoutStyle* parentStyle = 0);
+    StyleResolverState(Document&, const ElementResolveContext&, const LayoutStyle* parentStyle);
+    StyleResolverState(Document&, Element*, const LayoutStyle* parentStyle = 0);
     ~StyleResolverState();
 
     // In FontFaceSet and CanvasRenderingContext2D, we don't have an element to grab the document from.
@@ -83,9 +83,13 @@ public:
     const CSSAnimationUpdate* animationUpdate() { return m_animationUpdate.get(); }
     PassOwnPtrWillBeRawPtr<CSSAnimationUpdate> takeAnimationUpdate();
 
-    void setParentStyle(PassRefPtr<LayoutStyle> parentStyle) { m_parentStyle = parentStyle; }
-    const LayoutStyle* parentStyle() const { return m_parentStyle.get(); }
-    LayoutStyle* parentStyle() { return m_parentStyle.get(); }
+    void setParentStyle(PassRefPtr<LayoutStyle> parentStyle) { m_overrideParentStyle = parentStyle; }
+    const LayoutStyle* 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
@@ -128,7 +132,7 @@ public:
     // set these directly on the LayoutStyle w/o telling us. Presumably we'll
     // want to design a better wrapper around LayoutStyle for tracking these mutations
     // and separate it from StyleResolverState.
-    const FontDescription& parentFontDescription() { return m_parentStyle->fontDescription(); }
+    const FontDescription& parentFontDescription() { return parentStyle()->fontDescription(); }
 
     void setZoom(float f)
     {
@@ -162,7 +166,8 @@ private:
 
     // m_parentStyle is not always just ElementResolveContext::parentStyle,
     // so we keep it separate.
-    RefPtr<LayoutStyle> m_parentStyle;
+    const LayoutStyle* m_parentStyle;
+    RefPtr<LayoutStyle> m_overrideParentStyle;
 
     OwnPtrWillBeMember<CSSAnimationUpdate> m_animationUpdate;