[webui][ntp] Only show customize dialog focus outline in keyboard mode

Fixed: 1119748
Change-Id: I9e1ae79e2c495c6a784869baa77f2e7e9fedca8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401852
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805526}

chrome/browser/resources/new_tab_page/customize_dialog.html

@@ -119,7 +119,7 @@
     background-color: var(--ntp-hover-background-color);
   }
 
-  .menu-item:focus {
+  :host-context(.focus-outline-visible) .menu-item:focus {
     box-shadow: var(--ntp-focus-shadow);
   }