visualViewport should be [SameObject, Replaceable]

third_party/WebKit/Source/core/frame/Window.idl

 /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 113 matching lines @@
     [Replaceable] readonly attribute double pageYOffset;
     [RuntimeEnabled=CSSOMSmoothScroll] void scroll(optional ScrollToOptions options);
     void scroll(unrestricted double x, unrestricted double y);
     [RuntimeEnabled=CSSOMSmoothScroll] void scrollTo(optional ScrollToOptions options);
     void scrollTo(unrestricted double x, unrestricted double y);
     [RuntimeEnabled=CSSOMSmoothScroll] void scrollBy(optional ScrollToOptions options);
     void scrollBy(unrestricted double x, unrestricted double y);
 
     // Visual Viewport API
     // https://github.com/WICG/ViewportAPI
-    [RuntimeEnabled=VisualViewportAPI] readonly attribute VisualViewport? visualViewport;
+    [RuntimeEnabled=VisualViewportAPI, Replaceable, SameObject] readonly attribute VisualViewport? visualViewport;
 
     // client
     [Replaceable] readonly attribute long screenX;
     [Replaceable] readonly attribute long screenY;
     [Replaceable] readonly attribute long outerWidth;
     [Replaceable] readonly attribute long outerHeight;
     [Replaceable] readonly attribute double devicePixelRatio;
 
     // Selection API
     // http://w3c.github.io/selection-api/#extensions-to-window-interface
@@ skipping 64 matching lines @@
 // FIXME: make this typedef accurate once enough of http://crbug.com/240176
 // is in place.
 // FIXME: consider putting this typedef in an .idl file containing spec-wide
 // utility type definitions.
 typedef MessagePort Transferable;
 
 Window implements GlobalEventHandlers;
 Window implements WindowBase64;
 Window implements WindowEventHandlers;
 Window implements WindowTimers;