Introduce Node::containingTreeScope(), which asserts that the node's root is a tree scope

third_party/WebKit/Source/core/dom/Node.h

Index: third_party/WebKit/Source/core/dom/Node.h
diff --git a/third_party/WebKit/Source/core/dom/Node.h b/third_party/WebKit/Source/core/dom/Node.h
index a9ee2d12e383f75c545bde438b64b46baec4f808..fedf63e639b20271dd49a9d7fb12d86cfa38f785 100644
--- a/third_party/WebKit/Source/core/dom/Node.h
+++ b/third_party/WebKit/Source/core/dom/Node.h
@@ -471,6 +471,12 @@ public:
         return *m_treeScope;
     }
 
+    TreeScope& containingTreeScope() const
+    {
+        DCHECK(isInTreeScope());
+        return *m_treeScope;
+    }
+
     bool inActiveDocument() const;
 
     // Returns true if this node is associated with a shadow-including document and is in its associated document's