Have StyledMarkupSerializer work on the Composed tree

Source/core/editing/EditingStrategy.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/editing/EditingStrategy.h"
 
+#include "core/dom/shadow/ElementShadow.h"
 #include "core/editing/htmlediting.h"
 
 namespace blink {
 
 template <typename Traversal>
 bool EditingAlgorithm<Traversal>::isEmptyNonEditableNodeInEditable(const Node* node)
 {
     // Editability is defined the DOM tree rather than the composed tree. For example:
     // DOM:
     //   <host><span>unedittable</span><shadowroot><div ce><content /></div></shadowroot></host>
@@ skipping 120 matching lines @@
         if (n == childB)
             return 1; // A is after B
         n = Traversal::nextSibling(*n);
     }
 
     // Should never reach this point.
     ASSERT_NOT_REACHED();
     return 0;
 }
 
+bool EditingInComposedTreeStrategy::isSelectionBoundaryShadowHost(const Node& node)
+{
+    if (!node.isElementNode())
+        return false;
+    ElementShadow* shadow = toElement(node).shadow();
+    if (!shadow)
+        return false;
+    return shadow->youngestShadowRoot()->type() == ShadowRoot::UserAgentShadowRoot;
+}
+
 ContainerNode* EditingInComposedTreeStrategy::parentOrShadowHostNode(const Node& node)
 {
     return parent(node);
 }
 
 template class EditingAlgorithm<NodeTraversal>;
 template class EditingAlgorithm<ComposedTreeTraversal>;
 
 } // namespace blink