High DPI support for themes: Raw images for NTP attribution and background

chrome/browser/resources/new_incognito_tab_theme.css

 /* Copyright (c) 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: $2;
-  background-image: -webkit-image-set(
-    url(chrome://theme/IDR_THEME_NTP_BACKGROUND?$1) 1x);
+  background-image: url(chrome://theme/IDR_THEME_NTP_BACKGROUND?$1);
   background-position: $3;
   background-repeat: $5;
   height: 100%;
   overflow: auto;
 }
 
+#attribution-img {
+  content: url(chrome://theme/IDR_THEME_NTP_ATTRIBUTION?$1);
+}
+
 html[bookmarkbarattached='true'] {
   background-position: $4;
 }