DevTools: introduce screencast experiment.

Source/devtools/front_end/inspector.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 476 matching lines @@
 }
 
 .toolbar-item.close-left,
 .toolbar-item.close-right {
   display: -webkit-flex;
   -webkit-align-items: center;
   height: 100%;
   cursor: default;
 }
 
+#root {
+    position: absolute;
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
+}
+
 #main {
     position: absolute;
     z-index: 1;
     top: 26px;
     left: 0;
     right: 0;
     bottom: 0;
     overflow: hidden;
     background-color: white;
 }
@@ skipping 2468 matching lines @@
     pointer-events: auto;
     -webkit-user-select: text;
     font-size: 11px;
     cursor: text;
     line-height: 14px;
 }
 
 .elements-tree-outline li.parent::before {
     top: 0 !important;
 }