DevTools: [timeline tree view] render visual of percent in the background.

Source/devtools/front_end/timeline/timelinePanel.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 989 matching lines @@
 .timeline-tree-view .data-grid tr:not(.selected) .highlight {
     background-color: rgb(255, 230, 179);
 }
 
 .timeline-tree-view .data-grid tr:hover td:not(.bottom-filler-td) {
     background-color: rgba(0, 0, 0, 0.1);
 }
 
 .timeline-tree-view .data-grid td.numeric-column {
     text-align: right;
+    position: relative;
 }
 
 .timeline-tree-view .data-grid div.profile-multiple-values {
     float: right;
 }
 
 .timeline-tree-view .data-grid span.percent-column {
-    color: #999;
+    color: #888;
     width: 48px;
     display: inline-block;
 }
 
 .timeline-tree-view .data-grid tr.selected span {
     color: inherit;
 }
 
 .timeline-tree-view .data-grid .name-container {
     display: flex;
@@ skipping 10 matching lines @@
     margin: 0 4px 0 2px;
     border: 1px solid rgba(0, 0, 0, 0.1);
 }
 
 .timeline-tree-view .data-grid .name-container .activity-link {
     flex: auto;
     text-align: right;
     overflow: hidden;
     text-overflow: ellipsis;
 }
+
+.timeline-tree-view .data-grid .background-bar {
+    height: 15px;
+    background-color: hsla(0, 100%, 74%, 0.1);
+    position: absolute;
+    right: -2px;
+    bottom: 0px;
+    border-bottom: 1px solid hsl(0, 100%, 74%);
+}
+
+.timeline-tree-view .data-grid .selected .background-bar {
+    background-color: rgba(255,255,255,0.25);
+    border-bottom-color: transparent;
+}