[DevTools] Add header for OOPIF's elements trees.

third_party/WebKit/Source/devtools/front_end/elements/elementsPanel.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 502 matching lines @@
     position: absolute;
     right: 0;
     bottom: 0;
     visibility: hidden;
     background-color: rgba(255, 255, 255, 0.9);
 }
 
 .styles-pane:not(.is-editing-style) .styles-section.matched-styles:not(.read-only):hover .sidebar-pane-section-toolbar {
     visibility: visible;
 }
+
+.elements-tree-header {
+    height: 24px;
+    border-top: 1px solid #eee;
+    border-bottom: 1px solid #eee;
+    display: flex;
+    flex-direction: row;
+    align-items: center;
+}
+
+.elements-tree-header-frame {
+    margin-left: 6px;
+    margin-right: 6px;
+    flex: none;
+}