DevTools: report timeline progress while recording.

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 797 matching lines @@
     display: none;
 }
 
 .timeline-flamechart-view .flame-chart-main-pane .resources-divider-label {
     text-align: center;
 }
 
 .timeline-flamechart {
     overflow: hidden;
 }
+
+.timeline-progress-pane {
+    position: absolute;
+    top: 0px;
+    right: 0px;
+    bottom: 0px;
+    left: 0px;
+    color: #777;
+    background-color: rgba(255, 255, 255, 0.8);
+    font-size: 30px;
+    z-index: 500;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+}