Implement *multiply*() methods in DOMMatrix.

Source/core/dom/DOMMatrix.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.
 
 [
     Constructor,
     Constructor(DOMMatrixReadOnly other),
     // FIXME: Should implement more constructors (See: crbug.com/388780)
     RuntimeEnabled=GeometryInterfaces,
 ] interface DOMMatrix : DOMMatrixReadOnly {
@@ skipping 17 matching lines @@
     attribute unrestricted double m32;
     attribute unrestricted double m33;
     attribute unrestricted double m34;
     attribute unrestricted double m41;
     attribute unrestricted double m42;
     attribute unrestricted double m43;
     attribute unrestricted double m44;
 
     // FIXME: Should implement some methods (See: crbug.com/388780)
     // Mutable transform methods
+    DOMMatrix multiplySelf(DOMMatrix other);
+    DOMMatrix preMultiplySelf(DOMMatrix other);
     DOMMatrix translateSelf(unrestricted double tx,
                             unrestricted double ty,
                             optional unrestricted double tz = 0);
     DOMMatrix scaleSelf(unrestricted double scale,
                         optional unrestricted double ox = 0,
                         optional unrestricted double oy = 0);
     DOMMatrix scale3dSelf(unrestricted double scale,
                           optional unrestricted double ox = 0,
                           optional unrestricted double oy = 0,
                           optional unrestricted double oz = 0);
     DOMMatrix scaleNonUniformSelf(unrestricted double sx,
                                   optional unrestricted double sy = 1,
                                   optional unrestricted double sz = 1,
                                   optional unrestricted double ox = 0,
                                   optional unrestricted double oy = 0,
                                   optional unrestricted double oz = 0);
 };