Low risk non-nullable => nullable change

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 38 matching lines @@
     attribute DOMString status;
     [CrossOrigin, CallWith=ExecutionContext] void close();
     [CrossOrigin] readonly attribute boolean closed;
     void stop();
     [CrossOrigin, CallWith=ExecutionContext] void focus();
     [CrossOrigin] void blur();
 
     // other browsing contexts
     [Replaceable, CallWith=ScriptState, CrossOrigin] readonly attribute Window frames;
     [Replaceable, CrossOrigin] readonly attribute unsigned long length;
-    [Unforgeable, CrossOrigin] readonly attribute Window top;
+    [Unforgeable, CrossOrigin] readonly attribute Window? top;
     // FIXME: opener should be of type any.
     [CrossOrigin, Custom=Setter] attribute Window opener;
-    [Replaceable, CrossOrigin] readonly attribute Window parent;
+    [Replaceable, CrossOrigin] readonly attribute Window? parent;
     [CheckSecurity=ReturnValue, Custom=Getter] readonly attribute Element? frameElement;
     // FIXME: open() should have 4 optional arguments with defaults.
     [Custom] Window? open(DOMString url, DOMString target, optional DOMString features);
     // FIXME: These getters should not have [NotEnumerable].
     [NotEnumerable, CrossOrigin] getter Window (unsigned long index);
     [Custom, NotEnumerable, CrossOrigin] getter object (DOMString name);
 
     // the user agent
     [LogActivity=GetterOnly] readonly attribute Navigator navigator;
     [LogActivity=GetterOnly] readonly attribute ApplicationCache applicationCache;
@@ skipping 135 matching lines @@
 // FIXME: make this typedef accurate once enough of https://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;