Timeline: show impl-side frames on the Timeline overview

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 47 matching lines @@
 
 .timeline-frame-overview #timeline-overview-grid {
     display: none;
 }
 
 #timeline-overview-grid .resources-dividers-label-bar {
     pointer-events: auto;
 }
 
 .timeline-frame-overview .overview-grid-window,
-.timeline-frame-overview .overview-grid-dividers-background {
-    height: 100%;
+.timeline-frame-overview .overview-grid-dividers-background,
+.timeline-frame-overview .overview-grid-window-resizer {
+    height: 15px;
 }
 
 #timeline-overview-grid #resources-graphs {
     position: absolute;
     top: 0;
     left: 0;
     right: 0;
     height: 80px;
 }
 
@@ skipping 517 matching lines @@
 
 @-webkit-keyframes timeline_record_highlight {
     from {background-color: rgba(255, 255, 120, 1); }
     to { background-color: rgba(255, 255, 120, 0); }
 }
 
 .timeline-filters-header {
     flex: 0 0 23px;
     overflow: hidden;
 }