Restrict the types of nodes that could be assigned to a slot.

third_party/WebKit/Source/core/dom/NonDocumentTypeChildNode.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // https://dom.spec.whatwg.org/#interface-nondocumenttypechildnode
 
 [
     LegacyTreatAsPartialInterface,
     NoInterfaceObject, // Always used on target of 'implements'
 ] interface NonDocumentTypeChildNode {
     [PerWorldBindings] readonly attribute Element previousElementSibling;
     [PerWorldBindings] readonly attribute Element nextElementSibling;
-    [RuntimeEnabled=ShadowDOMV1, ImplementedAs=assignedSlotForBinding] readonly attribute HTMLSlotElement assignedSlot;
 };