DevTools: Add labels to the timeline overview strips

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 236 matching lines @@
 }
 
 .timeline-tree-item.selected .timeline-tree-item-expand-arrow {
     background-color: white;
 }
 
 #timeline-overview-panel .overview-strip {
     margin-top: 1px;
 }
 
+#timeline-overview-panel .overview-strip .timeline-overview-strip-placeholder {
+    position: absolute;
+    right: 0;
+    bottom: 1px;
+    color: #888;
+    padding: 0 4px;
+    font-size: 11px;
+    font-weight: bold;
+    text-transform: uppercase;
+    background-color: hsla(0, 0%, 100%, 0.6);
+}
+
 #timeline-overview-main-thread,
 #timeline-overview-framerate {
     flex-basis: 20px;
 }
 
 #timeline-overview-input,
 #timeline-overview-other-threads,
 #timeline-overview-responsiveness {
-    flex-basis: 8px;
+    flex-basis: 10px;
 }
 
 #timeline-overview-network {
-    flex-basis: 39px;
+    flex-basis: 35px;
 }
 
 #timeline-overview-memory {
     flex-basis: 20px;
 }
 
 #timeline-overview-frames {
     flex-basis: 120px;
 }
 
@@ skipping 599 matching lines @@
     cursor: pointer;
 }
 
 .timeline-overview-popover .frame .time {
     display: none;
 }
 
 .timeline-overview-popover .frame .thumbnail img {
     max-width: 200px;
 }