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

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

 /*
  * Copyright (C) 2006, 2007, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * 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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 54 matching lines @@
     [TreatNullAs=NullString, CustomElementCallbacks, RaisesException=Setter] attribute DOMString outerHTML;
     [CustomElementCallbacks, RaisesException, MeasureAs=InsertAdjacentHTML] void insertAdjacentHTML(DOMString position, DOMString text);
 
     // Shadow DOM
     // http://w3c.github.io/webcomponents/spec/shadow/#extensions-to-element-interface
     [RaisesException, CallWith=ScriptState, MeasureAs=ElementCreateShadowRoot] ShadowRoot createShadowRoot();
     [RuntimeEnabled=ShadowDOMV1, RaisesException, CallWith=ScriptState, MeasureAs=ElementAttachShadow] ShadowRoot attachShadow(ShadowRootInit shadowRootInitDict);
     NodeList getDestinationInsertionPoints();
     [PerWorldBindings, ImplementedAs=openShadowRoot] readonly attribute ShadowRoot? shadowRoot;
     [RuntimeEnabled=ShadowDOMV1, Reflect] attribute DOMString slot;
+    [RuntimeEnabled=ShadowDOMV1, ImplementedAs=assignedSlotForBinding] readonly attribute HTMLSlotElement assignedSlot;
 
     // Pointer Lock
     // https://dvcs.w3.org/hg/pointerlock/raw-file/default/index.html#extensions-to-the-element-interface
     [MeasureAs=ElementRequestPointerLock] void requestPointerLock();
 
     // CSSOM View Module
     // http://dev.w3.org/csswg/cssom-view/#extension-to-the-element-interface
     // FIXME: getClientRect() and getBoundingClientRect() should
     // return DOMRectList and DOMRect respectively.
     ClientRectList getClientRects();
@@ skipping 38 matching lines @@
     attribute EventHandler oncut;
     attribute EventHandler onpaste;
     attribute EventHandler onsearch;
     attribute EventHandler onselectstart;
     attribute EventHandler onwheel;
 };
 
 Element implements ParentNode;
 Element implements ChildNode;
 Element implements NonDocumentTypeChildNode;