Custom Elements in template should not be upgraded

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index f8b010f16f056201ec128efaef0792b3e61e6405..aa20b9ffb261ff1a34032d9568704e323b5e4513 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -993,7 +993,9 @@ PassRefPtrWillBeRawPtr<Node> Document::importNode(Node* importedNode, bool deep,
             if (!importContainerNodeChildren(oldElement, newElement, exceptionState))
                 return nullptr;
             if (isHTMLTemplateElement(*oldElement)
-                && !importContainerNodeChildren(toHTMLTemplateElement(oldElement)->content(), toHTMLTemplateElement(newElement)->content(), exceptionState))
+                && !ensureTemplateDocument().importContainerNodeChildren(
+                    toHTMLTemplateElement(oldElement)->content(),
+                    toHTMLTemplateElement(newElement)->content(), exceptionState))
                 return nullptr;
         }