NTPResourceCache don't add substitution for colorSectionBorder twice

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 f5a66984bd800553282f81ad3d753215bc25d841..56bb6889f340f5d55574c0a93ff031c250d9794e 100644
--- a/chrome/browser/resources/ntp4/new_tab_theme.css
+++ b/chrome/browser/resources/ntp4/new_tab_theme.css
@@ -78,11 +78,6 @@ body {
   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};
 }