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

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

Index: third_party/WebKit/Source/core/dom/ContainerNode.cpp
diff --git a/third_party/WebKit/Source/core/dom/ContainerNode.cpp b/third_party/WebKit/Source/core/dom/ContainerNode.cpp
index a57cc701b61a798ffe0e0e6583e7f8a9fae3f3dd..d7f3ac0c07bc72120e077f48712cfacdb70a4443 100644
--- a/third_party/WebKit/Source/core/dom/ContainerNode.cpp
+++ b/third_party/WebKit/Source/core/dom/ContainerNode.cpp
@@ -1354,7 +1354,7 @@ 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 = containingTreeScope().getElementById(id);
         if (!element)
             return nullptr;
         if (element->isDescendantOf(this))