Adding matrixTrasnform to DOMPointReadOnly interfaces as following spec.

third_party/WebKit/Source/core/dom/DOMPointReadOnly.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/#DOMPoint
 
 [
-    Constructor(unrestricted double x, unrestricted double y,
-                unrestricted double z, unrestricted double w),
+    Constructor(optional unrestricted double x = 0, optional unrestricted double y = 0,

[zino, 2017/01/03 16:16:55]

Please add some tests for this change?

[Byoungkwon Ko, 2017/01/03 16:35:39]

Done.

+                optional unrestricted double z = 0, optional unrestricted double w = 1),
     // FIXME: Exposed=(Window,Worker)
     RuntimeEnabled=GeometryInterfaces,
 ] interface DOMPointReadOnly {
     [NewObject] static DOMPointReadOnly fromPoint(optional DOMPointInit other);
 
     readonly attribute unrestricted double x;
     readonly attribute unrestricted double y;
     readonly attribute unrestricted double z;
     readonly attribute unrestricted double w;
 
     // FIXME: Implement matrixTransform.
     // DOMPoint matrixTransform(DOMMatrixReadOnly matrix);
     serializer = { attribute };
+    [RaisesException] DOMPoint matrixTransform(optional DOMMatrixInit matrix);
 };