MD Extensions polish: Add 64px margin at the bottom of shortcuts page.

Bug: 787229
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I173e9c72b269c69a18f9c7909cfc60a0158f4707
Reviewed-on: https://chromium-review.googlesource.com/806930Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521597}

chrome/browser/resources/md_extensions/keyboard_shortcuts.html

@@ -20,17 +20,18 @@
       }
 
       .shortcut-card {
+        @apply --cr-primary-text;
+        @apply --shadow-elevation-2dp;
+        background-color: white;
         margin: 0 auto 16px auto;
         max-width: var(--card-max-width);
         min-width: var(--card-min-width);
+        padding-bottom: 8px;
         width: 90%;
       }
 
-      .shortcut-card {
-        @apply --cr-primary-text;
-        @apply --shadow-elevation-2dp;
-        background-color: white;
-        padding-bottom: 8px;
+      .shortcut-card:last-of-type {
+        margin-bottom: 64px;
       }
 
       #container {