MD Downloads: Specify spacing between buttons

Spacing between buttons on the downloads page was not specified in the
styling, and was instead the result of whitespace that is removed when
using the legacyOptimizations: true flag in Polymer 2. Specify 8px
margin between buttons to match spacing used in other Web UI
paper-buttons.

Bug: 912488
Change-Id: Icca140d02fe85fbb8b847b67f6925da87e13b26f
Reviewed-on: https://chromium-review.googlesource.com/c/1373137Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616015}

chrome/browser/resources/md_downloads/item.html

@@ -229,6 +229,11 @@
         left: 16px;
         right: initial;
       }
+
+      paper-button.action-button,
+      paper-button.pause-button {
+        margin-inline-end: 8px;
+      }
     </style>
 
     <h3 id="date">[[computeDate_(data.hideDate,

chrome/browser/resources/md_downloads/item.js

@@ -270,7 +270,8 @@ cr.define('downloads', function() {
       if (this.data === undefined)
         return '';
 
-      return !this.isInProgress_ && this.data.resume ? 'action-button' : '';
+      return !this.isInProgress_ && this.data.resume ? 'action-button' :
+                                                       'pause-button';
     },
 
     /**