[Wallpaper Picker] Fixed custom wallpaper Context Menu z-index

chrome/browser/resources/chromeos/wallpaper_manager/css/wallpaper_manager.css

 /* Copyright (c) 2013 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. */
 
 html {
   height: 100%;
 }
 
 body {
   -webkit-app-region: drag;
@@ skipping 153 matching lines @@
   z-index: 1;
 }
 
 .image-picker .check {
   background-image: url('../images/ui/check.png');
   height: 32px;
   left: 38px;
   position: absolute;
   top: 18px;
   width: 32px;
-  z-index: 3;
+  z-index: 2;
 }
 
 ::-webkit-scrollbar {
   width: 6px;
 }
 
 ::-webkit-scrollbar-thumb {
   background: #aaa;
 }
 
@@ skipping 148 matching lines @@
   margin-right: 4px;
   opacity: 0.7;
   position: relative;
   top: 4px;
   width: 17px;
 }
 
 #surprise-me #checkbox.checked {
   background-image: url('../images/ui/checkbox_checked.png');
 }