[css-typed-om] replace is2D() method with attribute in CSSTransformValue.

third_party/WebKit/Source/core/css/cssom/CSSTransformValue.idl

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
     Constructor(),
     Constructor(sequence<CSSTransformComponent> transformComponents),
     Exposed=(Window,PaintWorklet),
     RuntimeEnabled=CSSTypedOM
 ] interface CSSTransformValue : CSSStyleValue {
     // https://github.com/w3c/css-houdini-drafts/issues/358
     readonly attribute unsigned long length;
     [ImplementedAs=componentAtIndex] getter CSSTransformComponent (unsigned long index);
 
     iterable<CSSTransformComponent>;
 
-    boolean is2D();
+    readonly attribute boolean is2D;
 };