MD Extensios polish: Add border-radius for shortcut cards.

Bug: 790755
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I853f3765273c34fbd057708c745b5bc807d8dc3b
Reviewed-on: https://chromium-review.googlesource.com/822736Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523678}

chrome/browser/resources/md_extensions/keyboard_shortcuts.html

@@ -21,6 +21,7 @@
         @apply --cr-primary-text;
         @apply --shadow-elevation-2dp;
         background-color: white;
+        border-radius: 2px;
         margin: 0 auto 16px auto;
         padding-bottom: 8px;
         width: var(--cr-toolbar-field-width);