[css-typed-om] styleMap should have SameObject extended attribute.

third_party/WebKit/Source/core/dom/Element.idl

 /*
  * Copyright (C) 2006, 2007, 2009 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 98 matching lines @@
     readonly attribute long clientLeft;
     readonly attribute long clientWidth;
     readonly attribute long clientHeight;
 
     // Scroll Customization API. See crbug.com/410974 for details.
     [RuntimeEnabled=ScrollCustomization] void setApplyScroll(ScrollStateCallback scrollStateCallback, NativeScrollBehavior nativeScrollBehavior);
     [RuntimeEnabled=ScrollCustomization] void setDistributeScroll(ScrollStateCallback scrollStateCallback, NativeScrollBehavior nativeScrollBehavior);
 
     // Typed OM
     // https://drafts.css-houdini.org/css-typed-om/#inline-stylepropertymap-objects
-    [RuntimeEnabled=CSSTypedOM] readonly attribute StylePropertyMap styleMap;
+    [RuntimeEnabled=CSSTypedOM, SameObject] readonly attribute StylePropertyMap styleMap;
 
     // Non-standard API
     [MeasureAs=ElementScrollIntoViewIfNeeded] void scrollIntoViewIfNeeded(optional boolean centerIfNeeded);
     [RaisesException, CallWith=ScriptState, MeasureAs=ElementCreateShadowRoot] ShadowRoot createShadowRoot();
     NodeList getDestinationInsertionPoints();
 
     // Experimental accessibility API
     [RuntimeEnabled=ComputedAccessibilityInfo] readonly attribute DOMString? computedRole;
     [RuntimeEnabled=ComputedAccessibilityInfo] readonly attribute DOMString? computedName;
 
     // Accessibility Object Model
     [RuntimeEnabled=AccessibilityObjectModel] readonly attribute AccessibleNode? accessibleNode;
 
     // Event handler attributes
     attribute EventHandler onbeforecopy;
     attribute EventHandler onbeforecut;
     attribute EventHandler onbeforepaste;
     attribute EventHandler oncopy;
     attribute EventHandler oncut;
     attribute EventHandler onpaste;
     attribute EventHandler onsearch;
     attribute EventHandler onselectstart;
     attribute EventHandler onwheel;
 };
 
 Element implements ParentNode;
 Element implements ChildNode;
 Element implements NonDocumentTypeChildNode;