Avoid reaching for StyleResolver::m_features from StyleBuilder.

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 5747efb42f1cfcec7c27ef6ec0b1670b30649dd8..c484bf59f0f1baaf49c6d6d8d91e3fa6b70613e5 100644
--- a/Source/core/css/resolver/StyleResolverState.h
+++ b/Source/core/css/resolver/StyleResolverState.h
@@ -76,6 +76,9 @@ public:
     bool applyPropertyToRegularStyle() const { return m_applyPropertyToRegularStyle; }
     bool applyPropertyToVisitedLinkStyle() const { return m_applyPropertyToVisitedLinkStyle; }
 
+    // Holds all attribute names found while applying "content" properties that contain an "attr()" value.
+    Vector<AtomicString>& contentAttrValues() { return m_contentAttrValues; }
+
     void setLineHeightValue(CSSValue* value) { m_lineHeightValue = value; }
     CSSValue* lineHeightValue() { return m_lineHeightValue; }
 
@@ -148,6 +151,7 @@ private:
     // CSSToStyleMap is a pure-logic class and only contains
     // a back-pointer to this object.
     CSSToStyleMap m_styleMap;
+    Vector<AtomicString> m_contentAttrValues;
 };
 
 } // namespace WebCore