MD Extensions: remove separator from extensions card

Bug: 842002
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Id343156de4540c33d1db130c9260e9b3fc861ddc
Reviewed-on: https://chromium-review.googlesource.com/1055818Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558098}

chrome/browser/resources/md_extensions/item.html

@@ -78,7 +78,7 @@
       #main {
         display: flex;
         flex: 1;
-        padding: 16px 20px 17px;
+        padding: 16px 20px;
       }
 
       #content {
@@ -141,7 +141,6 @@
       #button-strip {
         /* Avoid ripple from overlapping container. */
         -webkit-margin-end: 20px;
-        border-top: var(--cr-separator-line);
         box-sizing: border-box;
         flex-shrink: 0;
         height: var(--cr-section-min-height);