The DOMRectReadOnly implementation are different with spec.

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

 // Copyright 2014 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.
 
 // https://dev.w3.org/fxtf/geometry/#DOMRect
 
 [
-    Constructor(unrestricted double x, unrestricted double y,
-                unrestricted double width, unrestricted double height),
+    Constructor(optional unrestricted double x = 0, optional unrestricted double y = 0,
+                optional unrestricted double width = 0, optional unrestricted double height = 0),
     // FIXME: Exposed=(Window,Worker)
     RuntimeEnabled=GeometryInterfaces,
 ] interface DOMRectReadOnly {
     readonly attribute unrestricted double x;
     readonly attribute unrestricted double y;
     readonly attribute unrestricted double width;
     readonly attribute unrestricted double height;
     readonly attribute unrestricted double top;
     readonly attribute unrestricted double right;
     readonly attribute unrestricted double bottom;
     readonly attribute unrestricted double left;
 };