Add high dpi folder icons.

chrome/browser/resources/shared/css/tree.css

 /* Copyright (c) 2012 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.
  */
 
 tree {
   display: block;
   outline: none;
   overflow: auto;
 }
@@ skipping 109 matching lines @@
   background-repeat: no-repeat;
   white-space: pre;
 }
 
 html[dir=rtl] .tree-label {
   background-position: 100% 50%;
 }
 
 html[dir=rtl] .tree-label,
 html[dir=rtl] .tree-row[may-have-children] > .tree-label {
-  background-image:
-   url('../../../../../ui/resources/default_100_percent/folder_closed_rtl.png');
+  background-image: -webkit-image-set(
+      url('../../../../../ui/resources/default_100_percent/common/folder_closed_rtl.png') 1x,
+      url('../../../../../ui/resources/default_200_percent/common/folder_closed_rtl.png') 2x);
 }
 
 html[dir=rtl] .tree-item[expanded] > .tree-row > .tree-label {
-  background-image:
-     url('../../../../../ui/resources/default_100_percent/folder_open_rtl.png');
+  background-image: -webkit-image-set(
+     url('../../../../../ui/resources/default_100_percent/common/folder_open_rtl.png') 1x,
+     url('../../../../../ui/resources/default_200_percent/common/folder_open_rtl.png') 2x);
 }
 
 tree[icon-visibility=hidden] .tree-label {
   -webkit-padding-start: 0;
   background-image: none !important;
 }
 
 tree[icon-visibility=parent] .tree-label,
 tree[icon-visibility=parent] .tree-row[has-children=false] > .tree-label {
   background-image: none;
 }
 
 .tree-label,
 .tree-row[may-have-children] > .tree-label {
-  background-image:
-      url('../../../../../ui/resources/default_100_percent/folder_closed.png');
+  background-image: -webkit-image-set(
+      url('../../../../../ui/resources/default_100_percent/common/folder_closed.png') 1x,
+      url('../../../../../ui/resources/default_200_percent/common/folder_closed.png') 2x);
 }
 
 .tree-item[expanded] > .tree-row > .tree-label {
-  background-image:
-      url('../../../../../ui/resources/default_100_percent/folder_open.png');
+  background-image: -webkit-image-set(
+      url('../../../../../ui/resources/default_100_percent/common/folder_open.png') 1x,
+      url('../../../../../ui/resources/default_200_percent/common/folder_open.png') 2x);
 }
 
 /* We need to ensure that even empty labels take up space */
 .tree-label:empty:after {
   content: ' ';
   white-space: pre;
 }
 
 .tree-rename > .tree-row > .tree-label {
   -webkit-user-modify: read-write-plaintext-only;
@@ skipping 12 matching lines @@
 <if expr="not is_macosx">
   outline: none;
 </if>
   padding: 1px 7px 1px 1px;
 }
 
 html[dir=rtl] .tree-item[editing] input {
   margin: -2px -3px -2px -8px;
   padding: 1px 1px 1px 7px;
 }