Implement style invalidation tree walk for targeted style recalc upon class change.

Source/core/css/analyzer/DescendantInvalidationSet.h

Index: Source/core/css/analyzer/DescendantInvalidationSet.h
diff --git a/Source/core/css/analyzer/DescendantInvalidationSet.h b/Source/core/css/analyzer/DescendantInvalidationSet.h
index d434553661e626f80c764a32b5cc4d6755b37ff3..38e58b72d7279f0c974503fcfbb96bab03827e1a 100644
--- a/Source/core/css/analyzer/DescendantInvalidationSet.h
+++ b/Source/core/css/analyzer/DescendantInvalidationSet.h
@@ -57,8 +57,11 @@ public:
     void addId(const AtomicString& id);
     void addTagName(const AtomicString& tagName);
 
-    void setWholeSubtreeInvalid() { m_allDescendantsMightBeInvalid = true; };
+    // Appends the classes in this DescendantInvalidationSet to the vector.
+    void getClasses(Vector<AtomicString>& classes);
 
+    void setWholeSubtreeInvalid() { m_allDescendantsMightBeInvalid = true; };
+    bool wholeSubtreeInvalid() { return m_allDescendantsMightBeInvalid; }
 private:
     DescendantInvalidationSet();