Have StyledMarkupSerializer work on the Composed tree

Source/core/editing/EditingStrategy.h

 // 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.
 
 #ifndef EditingStrategy_h
 #define EditingStrategy_h
 
 #include "core/dom/NodeTraversal.h"
 #include "core/dom/shadow/ComposedTreeTraversal.h"
 
@@ skipping 23 matching lines @@
     using PositionIteratorType = PositionIteratorAlgorithm<EditingStrategy>;
     using PositionType = PositionAlgorithm<EditingStrategy>;
 };
 
 // Composed tree version of editing algorithm
 class EditingInComposedTreeStrategy : public EditingAlgorithm<ComposedTreeTraversal> {
 public:
     using PositionIteratorType = PositionIteratorAlgorithm<EditingInComposedTreeStrategy>;
     using PositionType = PositionAlgorithm<EditingInComposedTreeStrategy>;
 
+    // Returns true if the given node is considered as selection boundary.
+    static bool isSelectionBoundaryShadowHost(const Node&);
+
     // Don't use |parentOrShadowHostNode()| in composed tree specific algorithm.
     // This function is provided here for sharing algorithm with
     // |TextIteratorAlgorithm|, which handles shadow tree within in
     // DOM traversal.
     static ContainerNode* parentOrShadowHostNode(const Node&);
 };
 
 extern template class EditingAlgorithm<NodeTraversal>;
 extern template class EditingAlgorithm<ComposedTreeTraversal>;
 
 } // namespace blink
 
 #endif // EditingStrategy_h