DevTools: [Popover] fix css for popover scrollbar

Source/devtools/front_end/inspectorStyle.css

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2009 Anthony Ricaud <rik@webkit.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 2787 matching lines @@
 
 .custom-popup-horizontal-scroll ::-webkit-scrollbar-track-piece:horizontal:end {
     margin-right: 5px;
 }
 
 .custom-popup-horizontal-scroll ::-webkit-scrollbar-track-piece:horizontal:end:corner-present {
     margin-right: 4px;
 }
 
 .custom-popup-horizontal-scroll ::-webkit-scrollbar-track-piece:horizontal:decrement {
-    -webkit-border-image: url(Images/trackHoriz.png) 0 11 0 11;
+    -webkit-border-image: url(Images/thumbHoriz.png) 0 11 0 11;
     border-color: transparent;
     border-width: 0 0 0 11px;
 }
 
 .custom-popup-horizontal-scroll ::-webkit-scrollbar-track-piece:horizontal:increment {
-    -webkit-border-image: url(Images/trackHoriz.png) 0 11 0 11;
+    -webkit-border-image: url(Images/thumbHoriz.png) 0 11 0 11;
     border-color: transparent;
     border-width: 0 11px 0 0;
 }
 
 /* Custom Vertical Scrollbar Styles */
 
 .custom-popup-vertical-scroll ::-webkit-scrollbar:vertical:corner-present {
     border-bottom-width: 0;
 }
 
@@ skipping 101 matching lines @@
 }
 
 .viewport-control-gap-element {
     color: transparent;
 }
 
 label.checkbox-with-label {
     display: flex;
     -webkit-user-select: none;
 }