Timeline: make frames in TimelineView selectable

Source/devtools/front_end/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 521 matching lines @@
     pointer-events: none;
 }
 
 .timeline-frame-container {
     height: 19px;
     overflow: hidden;
     background-color: rgb(220, 220, 220);
     opacity: 0.8;
     color: black;
     text-align: center;
-    padding-top: 3px;
     z-index: 220;
     pointer-events: auto;
 }
 
 .timeline-frame-strip {
     position: absolute;
     height: 19px;
+    padding-top: 3px;
+}
+
+.timeline-frame-strip.selected {
+    background-color: rgb(56, 121, 217);
+    color: white;
 }
 
 #timeline-grid-header {
     pointer-events: none;
     height: 19px;
 }
 
 #timeline-graph-records-header {
     pointer-events: none;
     height: 19px;
@@ skipping 217 matching lines @@
     z-index: 500;
     display: flex;
     justify-content: center;
     align-items: center;
 }
 
 .revealable-link {
     text-decoration: underline;
     cursor: pointer;
 }