[NTP] Custom link edit dialog match GM2 webUI spec

Minor change on custom link edit input label and text field background
color css setting according to GM2 webUI spec.

Bug: 873721
Change-Id: I6770ce6eedf4a4e5ec6cb273e34136d30ee17f58
Reviewed-on: https://chromium-review.googlesource.com/1174870Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583001}

chrome/browser/resources/local_ntp/custom_links_edit.css

@@ -46,7 +46,7 @@ body {
 }
 
 .field-title {
-  color: rgb(154, 160, 166);
+  color: rgb(95, 99, 104);
   font-size: 10px;
   font-weight: 500;
   margin-bottom: 4px;
@@ -57,7 +57,7 @@ body {
 }
 
 input {
-  background-color: rgb(232, 234, 237);
+  background-color: rgb(241, 243, 244);
   border: none;
   border-radius: 4px;
   caret-color: rgb(26, 115, 232);