Add the NonElementParentNode interface

Source/core/dom/DocumentFragment.idl

Index: Source/core/dom/DocumentFragment.idl
diff --git a/Source/core/dom/DocumentFragment.idl b/Source/core/dom/DocumentFragment.idl
index 005032a254e262464d636cccd1c2edcc3782e932..e4df24ae4991a85c533aad4e147c5b60da1ececa 100644
--- a/Source/core/dom/DocumentFragment.idl
+++ b/Source/core/dom/DocumentFragment.idl
@@ -17,12 +17,13 @@
  * Boston, MA 02110-1301, USA.
  */
 
+// https://dom.spec.whatwg.org/#interface-documentfragment
+
 [
     Constructor,
     ConstructorCallWith=Document,
 ] interface DocumentFragment : Node {
-    // NonElementParentNode API.
-    Element getElementById(DOMString elementId);
 };
 
 DocumentFragment implements ParentNode;
+DocumentFragment implements NonElementParentNode;