[css-typed-om] make all attributes of CSSScale mutable.

third_party/WebKit/Source/core/css/cssom/CSSScale.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(double x, double y),
     Constructor(double x, double y, double z),
     Exposed=(Window,PaintWorklet),
     RuntimeEnabled=CSSTypedOM
 ] interface CSSScale : CSSTransformComponent {
-    readonly attribute double x;
-    readonly attribute double y;
-    readonly attribute double z;
+    attribute double x;
+    attribute double y;
+    attribute double z;
 };