MD Downloads: remove per-item scrollbarWidth handling

chrome/browser/resources/md_downloads/manager.css

 /* Copyright 2015 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 :host {
   display: flex;
   flex: 1 0;
   flex-direction: column;
   height: 100%;
 }
 
 @media screen and (max-width: 1024px) {
   :host {
     flex-basis: 670px;  /* 622 card width + 24 left margin + 24 right margin. */
   }
 }
 
 #panel {
   --paper-header-panel-standard-container: {
     display: flex;
+    overflow-y: overlay;
   };
 }
 
 /* TODO(dbeam): upgrade to 1.0.4 to use --paper-header-panel-shadow.
  * https://github.com/PolymerElements/paper-header-panel/pull/39 */
 #panel /deep/ #dropShadow {
   display: none;
 }
 
 #no-downloads,
@@ skipping 13 matching lines @@
   font-size: 123.1%;
   font-weight: 500;
   justify-content: center;
 }
 
 #no-downloads .illustration {
   content: -webkit-image-set(url(1x/no_downloads.png) 1x,
                              url(2x/no_downloads.png) 2x);
   margin-bottom: 32px;
 }