Settings WebUI: Add bottom padding to menu to make room for focus ring

Bug: 962357
Change-Id: I834c12978f37ddba38c8c3be70ff34e5ba139823
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611139
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659343}

chrome/browser/resources/settings/settings_menu/settings_menu.html

@@ -16,6 +16,7 @@
       :host {
         box-sizing: border-box;
         display: block;
+        padding-bottom: 2px;
         padding-top: 8px;
       }