Implement referrerpolicy attr for iframes

Source/core/html/HTMLIFrameElement.idl

 /*
  * Copyright (C) 2006, 2007, 2009, 2010 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 14 matching lines @@
     [Reflect] attribute DOMString srcdoc;
     [Reflect] attribute DOMString name;
     [PutForwards=value] readonly attribute DOMSettableTokenList sandbox;
     // Note: The seamless attribute was once supported, but was removed.
     [Reflect] attribute boolean allowFullscreen;
     [Reflect] attribute DOMString width;
     [Reflect] attribute DOMString height;
     [CheckSecurity=Node] readonly attribute Document? contentDocument;
     readonly attribute Window? contentWindow;
     [CheckSecurity=Node, RaisesException] Document? getSVGDocument();
+    [RuntimeEnabled=ReferrerPolicyAttribute, Reflect, ReflectOnly=("","no-referrer","origin","no-referrer-when-downgrade","origin-when-cross-origin","unsafe-url"), ReflectMissing="", ReflectInvalid=""] attribute DOMString referrerpolicy;
+
 
     // obsolete members
     // https://html.spec.whatwg.org/#HTMLIFrameElement-partial
     [Reflect] attribute DOMString align;
     [Reflect] attribute DOMString scrolling;
     [Reflect] attribute DOMString frameBorder;
     [Reflect, URL] attribute DOMString longDesc;
 
     [Reflect, TreatNullAs=EmptyString] attribute DOMString marginHeight;
     [Reflect, TreatNullAs=EmptyString] attribute DOMString marginWidth;
 };