Implement *multiply*() methods in DOMMatrix.

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 18 matching lines @@
     readonly attribute unrestricted double m41;
     readonly attribute unrestricted double m42;
     readonly attribute unrestricted double m43;
     readonly attribute unrestricted double m44;
 
     readonly attribute boolean is2D;
     readonly attribute boolean isIdentity;
 
     // FIXME: Should implement some methods (See: crbug.com/388780)
     // Immutable transform methods
+    DOMMatrix multiply(DOMMatrix other);
     DOMMatrix translate(unrestricted double tx,
                         unrestricted double ty,
                         optional unrestricted double tz = 0);
     DOMMatrix scale(unrestricted double scale,
                     optional unrestricted double ox = 0,
                     optional unrestricted double oy = 0);
     DOMMatrix scale3d(unrestricted double scale,
                       optional unrestricted double ox = 0,
                       optional unrestricted double oy = 0,
                       optional unrestricted double oz = 0);
     DOMMatrix scaleNonUniform(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);
     Float32Array toFloat32Array();
     Float64Array toFloat64Array();
 };