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

chrome/browser/resources/ntp4/new_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;  /* COLOR_NTP_BACKGROUND */
-  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;
 }
 
+#attribution-img {
+  content: url(chrome://theme/IDR_THEME_NTP_ATTRIBUTION?$1);
+}
+
 html[bookmarkbarattached='true'] {
   background-position: $4;
 }
 
 body {
   color: $8;  /* COLOR_NTP_TEXT */
   height: 100%;
   overflow: auto;
 }
 
@@ skipping 77 matching lines @@
 }
 
 .thumbnail-wrapper {
   /* This shows through at the (rounded) thumbnail's corners. */
   background-color: $11;  /* COLOR_NTP_SECTION_BORDER */
 }
 
 .filler .thumbnail {
   border-color: $2;  /* COLOR_NTP_BACKGROUND */
 }