Implement flip*() methods in DOMMatrixReaOnly.

Source/core/dom/DOMMatrixReadOnly.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.
 
 [
     GarbageCollected,
     RuntimeEnabled=GeometryInterfaces,
 ] interface DOMMatrixReadOnly {
     // These attributes are simple aliases for certain elements of the 4x4 matrix
     readonly attribute unrestricted double a;
@@ skipping 46 matching lines @@
                      optional unrestricted double oy = 0);
     DOMMatrix rotateFromVector(unrestricted double x,
                                unrestricted double y);
     DOMMatrix rotateAxisAngle(unrestricted double x,
                               unrestricted double y,
                               unrestricted double z,
                               unrestricted double angle);
     DOMMatrix skewX(unrestricted double sx);
     DOMMatrix skewY(unrestricted double sy);
     DOMMatrix multiply(DOMMatrix other);
+    DOMMatrix flipX();
+    DOMMatrix flipY();
 
     Float32Array toFloat32Array();
     Float64Array toFloat64Array();
 };