DevTools: Show asynchronous call stacks on frontend.

Source/devtools/front_end/sidebarPane.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 70 matching lines @@
     background-clip: padding;
     margin-top: -1px;
 }
 
 .sidebar-pane-title:active {
     background-color: rgb(204, 204, 204);
     border-top: 1px solid rgb(178, 178, 178);
     border-bottom: 1px solid rgb(178, 178, 178);
 }
 
+.placard-group::before,
 .sidebar-pane-title::before {
     background-image: url(Images/statusbarButtonGlyphs.png);
     background-size: 320px 120px;
     opacity: 0.5;
     float: left;
     width: 11px;
     height: 11px;
     margin-right: 2px;
     content: "a";
     color: transparent;
     position: relative;
     top: 3px;
 }
 
 @media (-webkit-min-device-pixel-ratio: 1.5) {
+.placard-group::before,
 .sidebar-pane-title::before {
     background-image: url(Images/statusbarButtonGlyphs2x.png);
 }
 } /* media */
 
+.placard-group::before,
 .sidebar-pane-title::before {
     background-position: -4px -96px;
 }
 
+.placard-group.expanded::before,
 .sidebar-pane-title.expanded::before {
     background-position: -20px -96px;
 }
 
 .sidebar-pane-toolbar {
     line-height: 18px;
     left: 0;
     right: 4px;
     top: 0;
     height: 20px;
@@ skipping 51 matching lines @@
 
 .sidebar-pane-subtitle input,
 .section > .header input[type=checkbox] {
     height: 1em;
     width: 1em;
     margin-left: 0;
     margin-top: 0;
     margin-bottom: 0.25em;
     vertical-align: bottom;
 }