NTPResourceCache don't add substitution for colorSectionBorder twice

chrome/browser/resources/ntp4/new_tab_theme.css

 /* Copyright 2012 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. */
 
 html {
   background-attachment: fixed;
   background-color: $i18n{colorBackground};
   background-image: url(chrome://theme/IDR_THEME_NTP_BACKGROUND?$i18n{themeId});
   background-position: $i18n{backgroundBarDetached};
   background-repeat: $i18n{backgroundTiling};
@@ skipping 60 matching lines @@
       rgba($i18n{colorSectionBorder}, 0.2),
       rgba($i18n{colorSectionBorder}, 0.3) 20%,
       rgba($i18n{colorSectionBorder}, 0.3) 80%,
       rgba($i18n{colorSectionBorder}, 0.2));
 }
 
 .dot input:focus {
   background-color: $i18n{colorBackground};
 }
 
-.thumbnail-wrapper {
-  /* This shows through at the (rounded) thumbnail's corners. */
-  background-color: $i18n{colorSectionBorder};
-}
-
 .filler .thumbnail {
   border-color: $i18n{colorBackground};
 }