Have StyledMarkupSerializer work on the Composed tree

Source/core/editing/EditingStrategy.cpp

Index: Source/core/editing/EditingStrategy.cpp
diff --git a/Source/core/editing/EditingStrategy.cpp b/Source/core/editing/EditingStrategy.cpp
index 6f97a2e6fd8944650ce8bd737a6f85ad665b34fd..c3e9590b88d5f18a9d4782f5b814fedbc803ef97 100644
--- a/Source/core/editing/EditingStrategy.cpp
+++ b/Source/core/editing/EditingStrategy.cpp
@@ -5,6 +5,7 @@
 #include "config.h"
 #include "core/editing/EditingStrategy.h"
 
+#include "core/dom/shadow/ElementShadow.h"
 #include "core/editing/htmlediting.h"
 
 namespace blink {
@@ -145,6 +146,16 @@ short EditingAlgorithm<Traversal>::comparePositions(Node* containerA, int offset
     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);