MD Extension: change margin-end to padding-end

Bug: 842582
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I8b64c868ef6a4437116ebb2a363757a77b8674d3
Reviewed-on: https://chromium-review.googlesource.com/1067487Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560781}

chrome/browser/resources/md_extensions/item.html

@@ -139,8 +139,7 @@
       }
 
       #button-strip {
-        /* Avoid ripple from overlapping container. */
-        -webkit-margin-end: 20px;
+        -webkit-padding-end: 20px;
         box-sizing: border-box;
         flex-shrink: 0;
         height: var(--cr-section-min-height);