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

third_party/WebKit/Source/core/dom/ContainerNode.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2013 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 1448 matching lines @@
     DCHECK(isHTMLFormElement(this) || isHTMLFieldSetElement(this));
     CollectionType type = onlyMatchImgElements ? RadioImgNodeListType : RadioNodeListType;
     return ensureCachedCollection<RadioNodeList>(type, name);
 }
 
 Element* ContainerNode::getElementById(const AtomicString& id) const
 {
     if (isInTreeScope()) {
         // Fast path if we are in a tree scope: call getElementById() on tree scope
         // and check if the matching element is in our subtree.
-        Element* element = treeScope().getElementById(id);
+        Element* element = rootTreeScope().getElementById(id);
         if (!element)
             return nullptr;
         if (element->isDescendantOf(this))
             return element;
     }
 
     // Fall back to traversing our subtree. In case of duplicate ids, the first element found will be returned.
     for (Element& element : ElementTraversal::descendantsOf(*this)) {
         if (element.getIdAttribute() == id)
             return &element;
@@ skipping 19 matching lines @@
         return true;
 
     if (node->isElementNode() && toElement(node)->shadow())
         return true;
 
     return false;
 }
 #endif
 
 } // namespace blink