[cros] Add attribution (author & website) strings to stock user images.

chrome/browser/resources/options2/chromeos/set_wallpaper_options.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. */
 
 #wallpaper-title span {
   -webkit-padding-start: 17px;
   padding-top: 6px;
 }
 
 #set-wallpaper-page .content-area {
@@ skipping 17 matching lines @@
 
 .bottom-bar {
   -webkit-box-orient: vertical;
   display: -webkit-box;
 }
 
 .bottom-bar .spacer {
   -webkit-box-flex: 1;
 }
 
-.image-attr {
+.wallpaper-image-attr {
   -webkit-padding-start: 34px;
   line-height: 26px;
   padding-top: 14px;
 }
 
-#author-website {
+#wallpaper-author-website {
   -webkit-padding-start: 5px;
 }
 
 #wallpaper-action-area {
   -webkit-padding-start: 34px;
   padding-top: 0;
 }