Have StyledMarkupSerializer work on the Composed tree

Source/core/editing/EditingStrategy.h

Index: Source/core/editing/EditingStrategy.h
diff --git a/Source/core/editing/EditingStrategy.h b/Source/core/editing/EditingStrategy.h
index d68f1af30f2ede7fba30c3d9f8aaa652ea349b23..fab04794c992d7087255d42dac12f3e2e2fdc8de 100644
--- a/Source/core/editing/EditingStrategy.h
+++ b/Source/core/editing/EditingStrategy.h
@@ -41,6 +41,9 @@ 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