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

chrome/browser/resources/ntp4/new_tab_theme.css

Index: chrome/browser/resources/ntp4/new_tab_theme.css
diff --git a/chrome/browser/resources/ntp4/new_tab_theme.css b/chrome/browser/resources/ntp4/new_tab_theme.css
index 75163838582d658a2bccbd9feea28cad5c0102ae..adfeeb7c371adb0a171cad46d0e78d72345bc50d 100644
--- a/chrome/browser/resources/ntp4/new_tab_theme.css
+++ b/chrome/browser/resources/ntp4/new_tab_theme.css
@@ -5,12 +5,15 @@
 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;
 }