Ensure that every Document has a single, distinct template content ownerDocument

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index 4fa870bdcf8ccd59138f293e85766194fe2c40c7..05429d735f9985fd2cde4d38e1f87beaeb1f4342 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -1014,9 +1014,9 @@ public:
     const Vector<RefPtr<Element> >& topLayerElements() const { return m_topLayerElements; }
     HTMLDialogElement* activeModalDialog() const;
 
-    const Document* templateDocument() const;
+    // A non-null m_templateDocumentHost implies that |this| was created by ensureTemplateDocument().
+    bool isTemplateDocument() const { return !!m_templateDocumentHost; }
     Document& ensureTemplateDocument();
-    void setTemplateDocumentHost(Document* templateDocumentHost) { m_templateDocumentHost = templateDocumentHost; }
     Document* templateDocumentHost() { return m_templateDocumentHost; }
 
     void didAssociateFormControl(Element*);
@@ -1360,15 +1360,6 @@ inline void Document::notifyRemovePendingSheetIfNeeded()
         didRemoveAllPendingStylesheet();
 }
 
-inline const Document* Document::templateDocument() const
-{
-    // If DOCUMENT does not have a browsing context, Let TEMPLATE CONTENTS OWNER be DOCUMENT and abort these steps.
-    if (!m_frame)
-        return this;
-
-    return m_templateDocument.get();
-}
-
 inline bool Document::shouldOverrideLegacyDescription(ViewportDescription::Type origin)
 {
     // The different (legacy) meta tags have different priorities based on the type