Consolidate rendering of gray segmented spinner.

chrome/browser/resources/chromeos/login/spinner.css

-/* Copyright (c) 2011 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.
- */
-
-@-webkit-keyframes spinner {
-  /* There are 13 animation steps, the numbers below space them evenly. Note
-   * that a steps(13) declaration produces off-by-one offsets for some frames,
-   * so for best results we specify positions explicity and use step-end. */
-  0%           { background-position: 286px 0px; }
-  7.6923076%   { background-position: 264px 0px; }
-  15.3846153%  { background-position: 242px 0px; }
-  23.0769230%  { background-position: 220px 0px; }
-  30.7692307%  { background-position: 198px 0px; }
-  38.4615384%  { background-position: 176px 0px; }
-  46.1538461%  { background-position: 154px 0px; }
-  53.8461538%  { background-position: 132px 0px; }
-  61.5384615%  { background-position: 110px 0px; }
-  69.2307692%  { background-position: 88px  0px; }
-  76.9230769%  { background-position: 66px  0px; }
-  84.6153846%  { background-position: 44px  0px; }
-  92.3076923%  { background-position: 22px  0px; }
-  100.0000000% { background-position: 0px   0px; }
-}
-
-.spinner {
-  -webkit-animation: spinner 1s step-end infinite;
-  background-image: url('chrome://theme/IDR_SPINNER');
-  height: 22px;
-  width: 22px;
-}