StyleResolver should update RuleSets lazily.

Source/core/css/resolver/StyleResolver.h

Index: Source/core/css/resolver/StyleResolver.h
diff --git a/Source/core/css/resolver/StyleResolver.h b/Source/core/css/resolver/StyleResolver.h
index 76bd3075b0e4371cc3b8ddf042f85b2a8c3a2202..08aeedd6a69ef2c7b3a3d7b1615c70fbcf4d1d47 100644
--- a/Source/core/css/resolver/StyleResolver.h
+++ b/Source/core/css/resolver/StyleResolver.h
@@ -39,6 +39,7 @@
 #include "wtf/Deque.h"
 #include "wtf/HashMap.h"
 #include "wtf/HashSet.h"
+#include "wtf/ListHashSet.h"
 #include "wtf/RefPtr.h"
 #include "wtf/Vector.h"
 
@@ -215,6 +216,11 @@ public:
     void resetAtHostRules(const ContainerNode*);
     void finishAppendAuthorStyleSheets();
 
+    void lazyAppendAuthorStyleSheets(unsigned firstNew, const Vector<RefPtr<CSSStyleSheet> >&);
+    void removePendingAuthorStyleSheets(const Vector<RefPtr<CSSStyleSheet> >&);
+    void appendPendingAuthorStyleSheets();
+    bool hasPendingAuthorStyleSheets() const { return m_pendingStyleSheets.size() > 0 || m_needCollectFeatures; }
+
     DocumentRuleSets& ruleSets() { return m_ruleSets; }
     const DocumentRuleSets& ruleSets() const { return m_ruleSets; }
     SelectorFilter& selectorFilter() { return m_selectorFilter; }
@@ -263,6 +269,12 @@ public:
     InspectorCSSOMWrappers& inspectorCSSOMWrappers() { return m_inspectorCSSOMWrappers; }
 
     const RuleFeatureSet& ruleFeatureSet() const { return m_features; }
+    const RuleFeatureSet& ensureRuleFeatureSet()
+    {
+        if (hasPendingAuthorStyleSheets())
+            appendPendingAuthorStyleSheets(); // slow path.
+        return m_features;
+    }
 
     StyleSharingList& styleSharingList() { return m_styleSharingList; }
 
@@ -292,6 +304,7 @@ private:
     void matchUARules(ElementRuleCollector&);
     void matchUserRules(ElementRuleCollector&, bool includeEmptyRules);
     void collectFeatures();
+    void resetRuleFeatures();
 
     bool fastRejectSelector(const RuleData&) const;
 
@@ -347,6 +360,8 @@ private:
 
     RefPtr<ViewportStyleResolver> m_viewportStyleResolver;
 
+    ListHashSet<CSSStyleSheet*, 16> m_pendingStyleSheets;
+
     ScopedStyleTree m_styleTree;
 
     // FIXME: The entire logic of collecting features on StyleResolver, as well astransferring them
@@ -354,6 +369,7 @@ private:
     RuleFeatureSet m_features;
     OwnPtr<RuleSet> m_siblingRuleSet;
     OwnPtr<RuleSet> m_uncommonAttributeRuleSet;
+    bool m_needCollectFeatures;
 
     InspectorCSSOMWrappers m_inspectorCSSOMWrappers;