Add [CachedAccessor] attribute to cache (almost) constant accessors (window.document).

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 18 matching lines @@
 // FIXME: explain all uses of [DoNotCheckSecurity]
 [
     CheckSecurity=Receiver,
     ImplementedAs=DOMWindow,
     PrimaryGlobal,
 ] interface Window : EventTarget {
     // the current browsing context
     // FIXME: The spec uses the WindowProxy type for this and many other attributes.
     [Unforgeable, DoNotCheckSecurity] readonly attribute Window window;
     [Replaceable, DoNotCheckSecurity] readonly attribute Window self;
-    [Unforgeable] readonly attribute Document document;
+    [Unforgeable, CachedAccessor] readonly attribute Document document;
     attribute DOMString name;
     [PutForwards=href, Unforgeable, DoNotCheckSecurity] readonly attribute Location location;
     readonly attribute History history;
     [Replaceable, MeasureAs=BarPropLocationbar] readonly attribute BarProp locationbar;
     [Replaceable, MeasureAs=BarPropMenubar] readonly attribute BarProp menubar;
     [Replaceable, MeasureAs=BarPropPersonalbar] readonly attribute BarProp personalbar;
     [Replaceable, MeasureAs=BarPropScrollbars] readonly attribute BarProp scrollbars;
     [Replaceable, MeasureAs=BarPropStatusbar] readonly attribute BarProp statusbar;
     [Replaceable, MeasureAs=BarPropToolbar] readonly attribute BarProp toolbar;
     attribute DOMString status;
@@ skipping 159 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;