bookmarks: unclip dnd separator.

Also removes white cruft on top and bottom of #list while scrolling.

R=arv@chromium.org
BUG=429591

Review URL: https://codereview.chromium.org/696383002

Cr-Commit-Position: refs/heads/master@{#302474}

chrome/browser/resources/bookmark_manager/css/bmm.css

@@ -260,9 +260,9 @@ list > .folder > .label,
   -webkit-padding-end: 5px;
   box-sizing: border-box;
   display: inline-block;
-  margin-bottom: 5px;
-  margin-top: 5px;
   min-width: 100%;
+  padding-bottom: 5px;
+  padding-top: 5px;
 }
 
 #tree {