Store :last/first-child state as restyle flags.

Source/core/dom/ContainerNode.h

Index: Source/core/dom/ContainerNode.h
diff --git a/Source/core/dom/ContainerNode.h b/Source/core/dom/ContainerNode.h
index 52c67e84f844d49852c644fde1c122b7ffef64bf..b883311724178a50c25cfdc1429627498a362f0e 100644
--- a/Source/core/dom/ContainerNode.h
+++ b/Source/core/dom/ContainerNode.h
@@ -50,8 +50,10 @@ enum DynamicRestyleFlags {
     ChildrenAffectedByIndirectAdjacentRules = 1 << 7,
     ChildrenAffectedByForwardPositionalRules = 1 << 8,
     ChildrenAffectedByBackwardPositionalRules = 1 << 9,
+    AffectedByFirstChildRules = 1 << 10,
+    AffectedByLastChildRules = 1 << 11,
 
-    NumberOfDynamicRestyleFlags = 10,
+    NumberOfDynamicRestyleFlags = 12,
 };
 
 enum SubtreeModificationAction {
@@ -147,6 +149,12 @@ public:
     bool childrenAffectedByBackwardPositionalRules() const { return hasRestyleFlag(ChildrenAffectedByBackwardPositionalRules); }
     void setChildrenAffectedByBackwardPositionalRules() { setRestyleFlag(ChildrenAffectedByBackwardPositionalRules); }
 
+    bool affectedByFirstChildRules() const { return hasRestyleFlag(AffectedByFirstChildRules); }
+    void setAffectedByFirstChildRules() { setRestyleFlag(AffectedByFirstChildRules); }
+
+    bool affectedByLastChildRules() const { return hasRestyleFlag(AffectedByLastChildRules); }
+    void setAffectedByLastChildRules() { setRestyleFlag(AffectedByLastChildRules); }
+
     // FIXME: These methods should all be renamed to something better than "check",
     // since it's not clear that they alter the style bits of siblings and children.
     void checkForChildrenAdjacentRuleChanges();