[NTP] Fix arrow key navigation when shortcut option is clicked

Explicitly remove using-mouse-nav. The keydown event was prevented from
propagating to the listener that removed the mouse navigation class.

Bug: 1005712
Change-Id: I30b8d9c025cf5f56805134eefd6918892c19ec62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1825729Reviewed-by: Kyle Milka <kmilka@chromium.org>
Commit-Queue: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700023}

chrome/browser/resources/local_ntp/customize.js

@@ -2039,6 +2039,7 @@ customize.initCustomBackgrounds = function(showErrorNotification) {
       // Handle arrow key navigation.
       event.preventDefault();
       event.stopPropagation();
+      richerPicker.classList.remove(customize.CLASSES.MOUSE_NAV);
       if (event.keyCode === forwardArrowKey) {
         mvOption.focus();
       } else if (event.keyCode === customize.KEYCODES.DOWN) {
@@ -2067,6 +2068,7 @@ customize.initCustomBackgrounds = function(showErrorNotification) {
       // Handle arrow key navigation.
       event.preventDefault();
       event.stopPropagation();
+      richerPicker.classList.remove(customize.CLASSES.MOUSE_NAV);
       if (event.keyCode === backArrowKey) {
         clOption.focus();
       } else if (
@@ -2093,6 +2095,7 @@ customize.initCustomBackgrounds = function(showErrorNotification) {
       // Handle arrow key navigation.
       event.preventDefault();
       event.stopPropagation();
+      richerPicker.classList.remove(customize.CLASSES.MOUSE_NAV);
       if (event.keyCode === backArrowKey ||
           event.keyCode === customize.KEYCODES.UP) {
         mvOption.focus();