Implement referrerpolicy attribute for img elements

Source/core/html/HTMLImageElement.idl

 /*
  * Copyright (C) 2006, 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,
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     [Reflect] attribute DOMString sizes;
     [Reflect, ReflectOnly=("anonymous","use-credentials"), ReflectEmpty="anonymous", ReflectInvalid="anonymous"] attribute DOMString? crossOrigin;
     [Reflect] attribute DOMString useMap;
     [Reflect] attribute boolean isMap;
     attribute long width;
     attribute long height;
     readonly attribute long naturalWidth;
     readonly attribute long naturalHeight;
     readonly attribute boolean complete;
     readonly attribute DOMString currentSrc;
+    [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/#HTMLImageElement-partial
     [Reflect] attribute DOMString name;
     [Reflect, URL] attribute DOMString lowsrc;
     [Reflect] attribute DOMString align;
     [Reflect] attribute long hspace;
     [Reflect] attribute long vspace;
     [Reflect, URL] attribute DOMString longDesc;
 
     [Reflect, TreatNullAs=EmptyString] attribute DOMString border;
 
     // CSSOM View Module
     // http://dev.w3.org/csswg/cssom-view/#extensions-to-the-htmlimageelement-interface
     [MeasureAs=HTMLImageElementX] readonly attribute long x;
     [MeasureAs=HTMLImageElementY] readonly attribute long y;
 };