Only process each StyleSheetContents once when collecting features during style resolution

Source/core/css/resolver/ScopedStyleTree.cpp

Index: Source/core/css/resolver/ScopedStyleTree.cpp
diff --git a/Source/core/css/resolver/ScopedStyleTree.cpp b/Source/core/css/resolver/ScopedStyleTree.cpp
index 1938fd79218a580acbccddf50bdac9e35a43fc99..3fb5a45ed95c31615436223493f525f2943475bf 100644
--- a/Source/core/css/resolver/ScopedStyleTree.cpp
+++ b/Source/core/css/resolver/ScopedStyleTree.cpp
@@ -34,6 +34,8 @@
 
 namespace WebCore {
 
+class StyleSheetContents;
+
 ScopedStyleResolver* ScopedStyleTree::ensureScopedStyleResolver(ContainerNode& scopingNode)
 {
     bool isNewEntry;
@@ -191,8 +193,9 @@ void ScopedStyleTree::popStyleCache(const ContainerNode& scopingNode)
 
 void ScopedStyleTree::collectFeaturesTo(RuleFeatureSet& features)
 {
+    HashSet<const StyleSheetContents*> visitedSharedStyleSheetContents;
     for (HashMap<const ContainerNode*, OwnPtr<ScopedStyleResolver> >::iterator it = m_authorStyles.begin(); it != m_authorStyles.end(); ++it)
-        it->value->collectFeaturesTo(features);
+        it->value->collectFeaturesTo(features, visitedSharedStyleSheetContents);
 }
 
 inline void ScopedStyleTree::reparentNodes(const ScopedStyleResolver* oldParent, ScopedStyleResolver* newParent)