DevTools: Show main thread as a stacked utilization chart on timeline overview.

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 231 matching lines @@
     pointer-events: auto;
 }
 
 .timeline-tree-item.selected .timeline-tree-item-expand-arrow {
     background-color: white;
 }
 
 #timeline-overview-events,
 #timeline-overview-memory,
 #timeline-overview-power {
-    flex: 0 0 69px;
+    flex: 0 0 79px;
     z-index: 160;
     width: 100%;
     margin-bottom: 1px;
 }
 
 #timeline-overview-memory {
     flex-basis: 20px;
 }
 
 #timeline-overview-frames {
-    flex-basis: 80px;
+    flex-basis: 100px;
 }
 
 #timeline-overview-pane {
     flex: auto;
     position: relative;
     overflow: hidden;
 }
 
 #timeline-overview-container {
     display: flex;
@@ skipping 588 matching lines @@
 .timeline-gap {
     flex: none;
 }
 
 .timeline-filmstrip-preview {
     margin-top: 10px;
     max-width: 200px;
     max-height: 200px;
     cursor: pointer;
 }