[NTP] Prevent autoscroll when down arrow is pressed on bg menu

Bug: 996098
Change-Id: Ia7486e26c3225ff40a8fb9af36520788db0b58e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1763342Reviewed-by: Gayane Petrosyan <gayane@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689186}

chrome/browser/resources/local_ntp/customize.js

@@ -1923,12 +1923,14 @@ customize.initCustomBackgrounds = function(showErrorNotification) {
 
   $(customize.IDS.BACKGROUNDS_MENU).onkeydown = function(event) {
     if (customize.arrowKeys.includes(event.keyCode)) {
+      event.preventDefault();
       $(customize.IDS.BACKGROUNDS_UPLOAD_ICON).focus();
     }
   };
 
   $(customize.IDS.BACKGROUNDS_IMAGE_MENU).onkeydown = function(event) {
     if (customize.arrowKeys.includes(event.keyCode)) {
+      event.preventDefault();
       document.querySelector('[id$="img_tile_0"]').focus();
     }
   };