Get rid of TreeBoundaryCrossingRules.

Source/core/dom/TreeScope.cpp

Index: Source/core/dom/TreeScope.cpp
diff --git a/Source/core/dom/TreeScope.cpp b/Source/core/dom/TreeScope.cpp
index 801eafbc6ec8c056517e19f50aa3d1c0749c8d40..131a6a5cc56329e724f72f838d24e9a3f760b7ac 100644
--- a/Source/core/dom/TreeScope.cpp
+++ b/Source/core/dom/TreeScope.cpp
@@ -60,6 +60,7 @@ TreeScope::TreeScope(ContainerNode& rootNode, Document& document)
     : m_rootNode(&rootNode)
     , m_document(&document)
     , m_parentTreeScope(&document)
+    , m_composedParent(nullptr)
 #if !ENABLE(OILPAN)
     , m_guardRefCount(0)
 #endif
@@ -568,11 +569,35 @@ void TreeScope::setNeedsStyleRecalcForViewportUnits()
     }
 }
 
+TreeScope* TreeScope::composedParent()
+{
+    if (rootNode().isDocumentNode())
+        return nullptr;
+    if (!m_composedParent) {
+        ShadowRoot& root = toShadowRoot(rootNode());
+        if (root.olderShadowRoot())
+            m_composedParent = root.olderShadowRoot();
+        else if (ContainerNode* host = rootNode().parentOrShadowHostNode())
+            m_composedParent = treeScopeInComposedTree(*host);
+    }
+    return m_composedParent;
+}
+
+TreeScope* TreeScope::treeScopeInComposedTree(const Node& node)
+{
+    for (Node* composedParent = ComposedTreeTraversal::parent(node); composedParent; composedParent = ComposedTreeTraversal::parent(*composedParent)) {
+        if (composedParent && isShadowHost(composedParent))
+            return toElement(composedParent)->shadow()->youngestShadowRoot();
+    }
+    return &node.document();
+}
+
 DEFINE_TRACE(TreeScope)
 {
     visitor->trace(m_rootNode);
     visitor->trace(m_document);
     visitor->trace(m_parentTreeScope);
+    visitor->trace(m_composedParent);
     visitor->trace(m_idTargetObserverRegistry);
     visitor->trace(m_selection);
     visitor->trace(m_elementsById);