Add delete context menu for custom wallpapers and some ui tweaks

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 {
   background-color: rgba(20, 20, 20, 0.93);
   color: #999;
   font-size: 80%;
   height: 100%;
   margin: 0;
   padding: 0;
 }
 
 .dialog-container {
   -webkit-box-orient: vertical;
-  border: 1px solid black;
   display: -webkit-box;
   overflow: hidden;
 }
 
 .dialog-topbar {
   -webkit-box-align: center;
   display: -webkit-box;
   height: 54px;
   overflow: hidden;
   padding: 0 15px;
@@ skipping 179 matching lines @@
 }
 
 #author-website {
   color: #999;
   overflow: hidden;
   white-space: nowrap;
   width: 240px;
 }
 
 /* [hidden] does display:none, but its priority is too low. */
-[hidden] {
+[hidden],
+body[custom] [visibleif]:not([visibleif~='custom']),
+body:not([custom]) [visibleif~='custom'] {
   display: none !important;
 }
 
 .progress-bar {
   background-color: rgba(255, 255, 255, 0.7);
   height: 6px;
   position: absolute;
   top: 62px;
   width: 100%;
 }
@@ skipping 70 matching lines @@
   height: 30px;
 }
 
 #wallpaper-selection-container input[type='file']::-webkit-file-upload-button {
   height: 30px;
 }
 
 #wallpaper-selection-container .remember-icon {
   content: url('../images/ui/remember.png');
 }