WIP (Introduce WTF::NonNullPtr<T>.)

Source/core/xml/XPathFunctions.cpp

Index: Source/core/xml/XPathFunctions.cpp
diff --git a/Source/core/xml/XPathFunctions.cpp b/Source/core/xml/XPathFunctions.cpp
index 94ddadda8838143f9ae8758e3619df71032c7985..b9147455b85857fd741bbb32437e2af9133825be 100644
--- a/Source/core/xml/XPathFunctions.cpp
+++ b/Source/core/xml/XPathFunctions.cpp
@@ -330,7 +330,7 @@ Value FunId::evaluate() const
         idList.append(str);
     }
 
-    TreeScope& contextScope = evaluationContext().node->treeScope();
+    NonNullPtr<TreeScope> contextScope = evaluationContext().node->treeScope();
     NodeSet result;
     HashSet<Node*> resultSet;
 
@@ -349,7 +349,7 @@ Value FunId::evaluate() const
 
         // If there are several nodes with the same id, id() should return the first one.
         // In WebKit, getElementById behaves so, too, although its behavior in this case is formally undefined.
-        Node* node = contextScope.getElementById(idList.substring(startPos, endPos - startPos));
+        Node* node = contextScope->getElementById(idList.substring(startPos, endPos - startPos));
         if (node && resultSet.add(node).isNewEntry)
             result.append(node);