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

Source/core/dom/ContainerNode.cpp

Index: Source/core/dom/ContainerNode.cpp
diff --git a/Source/core/dom/ContainerNode.cpp b/Source/core/dom/ContainerNode.cpp
index d70e39e2f626376c4ac1739efc79423c41ab39f7..c2d260c353280a4faada257076ef39f087cb4cf5 100644
--- a/Source/core/dom/ContainerNode.cpp
+++ b/Source/core/dom/ContainerNode.cpp
@@ -1289,20 +1289,18 @@ void ContainerNode::checkForSiblingStyleChanges(SiblingCheckType changeType, Nod
         ASSERT(changeType != FinishedParsingChildren);
         // Find our new first child element.
         Element* firstChildElement = ElementTraversal::firstChild(*this);
-        RenderStyle* firstChildElementStyle = firstChildElement ? firstChildElement->renderStyle() : nullptr;
 
         // Find the first element after the change.
         Element* elementAfterChange = nodeAfterChange->isElementNode() ? toElement(nodeAfterChange) : ElementTraversal::nextSibling(*nodeAfterChange);
-        RenderStyle* elementAfterChangeStyle = elementAfterChange ? elementAfterChange->renderStyle() : nullptr;
 
         // This is the element insertion as first child element case.
-        if (firstChildElement != elementAfterChange && elementAfterChangeStyle && elementAfterChangeStyle->firstChildState()) {
+        if (firstChildElement != elementAfterChange && elementAfterChange && elementAfterChange->affectedByFirstChildRules()) {
             ASSERT(changeType == SiblingElementInserted);
             elementAfterChange->setNeedsStyleRecalc(SubtreeStyleChange, StyleChangeReasonForTracing::create(StyleChangeReason::SiblingSelector));
         }
 
         // This is the first child element removal case.
-        if (changeType == SiblingElementRemoved && firstChildElement == elementAfterChange && firstChildElement && (!firstChildElementStyle || !firstChildElementStyle->firstChildState()))
+        if (changeType == SiblingElementRemoved && firstChildElement == elementAfterChange && firstChildElement && firstChildElement->affectedByFirstChildRules())
             firstChildElement->setNeedsStyleRecalc(SubtreeStyleChange, StyleChangeReasonForTracing::create(StyleChangeReason::SiblingSelector));
     }
 
@@ -1311,21 +1309,19 @@ void ContainerNode::checkForSiblingStyleChanges(SiblingCheckType changeType, Nod
     if (childrenAffectedByLastChildRules() && nodeBeforeChange) {
         // Find our new last child element.
         Element* lastChildElement = ElementTraversal::lastChild(*this);
-        RenderStyle* lastChildElementStyle = lastChildElement ? lastChildElement->renderStyle() : nullptr;
 
         // Find the last element before the change.
         Element* elementBeforeChange = nodeBeforeChange->isElementNode() ? toElement(nodeBeforeChange) : ElementTraversal::previousSibling(*nodeBeforeChange);
-        RenderStyle* elementBeforeChangeStyle = elementBeforeChange ? elementBeforeChange->renderStyle() : nullptr;
 
         // This is the element insertion as last child element case.
-        if (lastChildElement != elementBeforeChange && elementBeforeChangeStyle && elementBeforeChangeStyle->lastChildState()) {
+        if (lastChildElement != elementBeforeChange && elementBeforeChange && elementBeforeChange->affectedByLastChildRules()) {
             ASSERT(SiblingElementInserted);
             elementBeforeChange->setNeedsStyleRecalc(SubtreeStyleChange, StyleChangeReasonForTracing::create(StyleChangeReason::SiblingSelector));
         }
 
         // This is the last child element removal case. The parser callback case is similar to node removal as well in that we need to change the last child
         // to match now.
-        if ((changeType == SiblingElementRemoved || changeType == FinishedParsingChildren) && lastChildElement == elementBeforeChange && lastChildElement && (!lastChildElementStyle || !lastChildElementStyle->lastChildState()))
+        if ((changeType == SiblingElementRemoved || changeType == FinishedParsingChildren) && lastChildElement == elementBeforeChange && lastChildElement && lastChildElement->affectedByLastChildRules())
             lastChildElement->setNeedsStyleRecalc(SubtreeStyleChange, StyleChangeReasonForTracing::create(StyleChangeReason::SiblingSelector));
     }