Remove lines of CSS which have invalid property values.

ui/webui/resources/css/expandable_bubble.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. */
 
 .expandable-bubble {
   -webkit-border-image: url('chrome://theme/IDR_APP_NOTIFICATION_SMALL_BUBBLE')
                         5 5 7 6 stretch;
   -webkit-box-sizing: border-box;
   -webkit-user-select: none;
   border-width: 5px 5px 7px 6px;
@@ skipping 36 matching lines @@
 }
 
 .expandable-bubble[expanded] > .expandable-bubble-contents >
     .expandable-bubble-title {
   font-size: 13px;
   margin-bottom: 3px;
   margin-left: 0;
 }
 
 .expandable-bubble-close {
-  background-image: no-repeat 50% 50%;
   height: 16px;
   position: absolute;
   right: 0;
   top: 0;
   width: 16px;
   z-index: 2;
 }
 
 .expandable-bubble[expanded] {
   padding: 3px;
@@ skipping 14 matching lines @@
   background-image: -webkit-image-set(
       url('../../../resources/default_100_percent/close_2_hover.png') 1x,
       url('../../../resources/default_200_percent/close_2_hover.png') 2x);
 }
 
 .expandable-bubble-close:active {
   background-image: -webkit-image-set(
       url('../../../resources/default_100_percent/close_2_pressed.png') 1x,
       url('../../../resources/default_200_percent/close_2_pressed.png') 2x);
 }