DevTools: promote large views with overflow to layers in order to not repaint on scroll.

Source/devtools/front_end/sidebarPane.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 16 matching lines @@
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 .sidebar-pane {
     position: relative;
 }
 
 .sidebar-pane > .body {
     position: relative;
     display: none;
-    overflow-y: auto;
-    overflow-x: hidden;
 }
 
 .sidebar-pane > .body .info {
     text-align: center;
     font-style: italic;
     font-size: 90%;
     padding: 6px;
     color: #888;
 }
 
@@ skipping 137 matching lines @@
 
 .sidebar-pane-subtitle input,
 .section > .header input[type=checkbox] {
     height: 1em;
     width: 1em;
     margin-left: 0;
     margin-top: 0;
     margin-bottom: 0.25em;
     vertical-align: bottom;
 }