about:tracing support for TRACE_ASYNC_START/FINISH events.

chrome/browser/resources/tracing/timeline.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.
-*/
+ * 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.
+ */
+
+.timeline * {
+  -webkit-user-select: none;
+  cursor: default;
+}
 
 .timeline-drag-box {
   background-color: rgba(0, 0, 255, 0.25);
   border: 1px solid #000060
   font-size: 75%;
   position: fixed;
 }
 
 .timeline-thread-track {
   -webkit-box-orient: vertical;
   display: -webkit-box;
   padding: 1px 0;
+  position: relative;
+}
+
+.timeline-track-close-button {
+  left: 0px;
+  position: absolute;
+  top: 0px;
+}
+
+.timeline-track-button {
+  color: rgba(0,0,0,0.1);
+  font-size: 10px;
+  height: 12px;
+  text-align: center;
+  width: 12px;
+}
+.timeline-track-button:hover {
+  border: 1px solid rgba(0, 0, 0, 0.2);
+  border-radius: 25%;
+  box-shadow: 0 0 .05em rgba(0,0,0,0.4);
+  color: rgba(0,0,0,0.5);
 }
 
 .timeline-thread-track:not(:first-child) {
   border-top: 1px solid #D0D0D0;
 }
 
 .timeline-canvas-based-track {
   -webkit-box-orient: horizontal;
   -webkit-box-align: stretch;
   background-color: white;
@@ skipping 26 matching lines @@
 .timeline-viewport-track {
   height: 12px;
 }
 
 .timeline-slice-track {
   height: 18px;
 }
 
 .timeline-counter-track {
   height: 30px;
+  position: relative;
 }