Replace TOUCH_UI condition in WebUI with dynamic flag.

chrome/browser/resources/chromeos/login/screen_gaia_signin.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.
  */
 
 #gaia-signin {
   -webkit-box-align: center;
   -webkit-box-pack: center;
   display: -webkit-box;
   height: 470px;
 }
 
 #signin-right {
   font-size: 12px;
   position: absolute;
   top: 60px;
   width: 200px;
 }
 
-/*  In TOUCH_UI builds, make the links easier to touch. */
-<if expr="pp_ifdef('touchui')">
-#signin-right {
+/*  For touch-optimized UI, make the links bigger. */
+html[touch-optimized] #signin-right {
   font-size: 14px;
 }
-</if>
 
 html[dir=ltr] #signin-right {
   right: -150px;  /* ('gaia-sign'.width - 'signin-frame'.width) / 2 -
                      desiredGap - 'signin-right'.width =
                      (640 - 340) / 2 - 100 - 200 = -150
                    */
 }
 
 html[dir=rtl] #signin-right {
   left: -150px;
@@ skipping 20 matching lines @@
 #gaia-loading {
   -webkit-box-align: center;
   -webkit-box-pack: center;
   bottom: 0;
   display: -webkit-box;
   left: 0;
   position: absolute;
   right: 0;
   top: 0;
 }