Inconsistent use of [x] close panel icon. Code changes.

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 56 matching lines @@
   padding: 3px;
   z-index: 3;  /* One higher then the close button on an unexpanded bubble. */
 }
 
 .expandable-bubble[expanded] > .expandable-bubble-close {
   z-index: 4;
 }
 
 .expandable-bubble-close {
   background-image: -webkit-image-set(
-      url('../../../resources/default_100_percent/close_bar.png') 1x,
-      url('../../../resources/default_200_percent/close_bar.png') 2x);
+      url('../../../resources/default_100_percent/close_2.png') 1x,
+      url('../../../resources/default_200_percent/close_2.png') 2x);
 }
 
 .expandable-bubble-close:hover {
   background-image: -webkit-image-set(
-      url('../../../resources/default_100_percent/close_bar_hover.png') 1x,
-      url('../../../resources/default_200_percent/close_bar_hover.png') 2x);
+      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_bar_pressed.png') 1x,
-    url('../../../resources/default_200_percent/close_bar_pressed.png') 2x);
+      url('../../../resources/default_100_percent/close_2_pressed.png') 1x,
+      url('../../../resources/default_200_percent/close_2_pressed.png') 2x);
 }