DevTools: migrate sidebar pane's titleElement to use Toolbar.

Source/devtools/front_end/network/networkPanel.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 22 matching lines @@
 
 .network-close-button {
     margin: auto -1px auto 4px;
 }
 
 .network-details-view-tall-header {
     margin-top: 8px;
 }
 
 .network-item-view {
-    display: none;
+    display: flex;
     background: white;
 }
 
-.network-item-view.visible {
-    display: flex;
-}
-
 .resource-timing-view {
-    display: none;
+    display: block;
     margin: 6px;
     color: rgb(30%, 30%, 30%);
 }
 
 /* Network timing is shared between popover and network item view pane */
 
 .network-timing-table {
     width: 380px;
     border-spacing: 0;
     padding-left: 10px;
@@ skipping 96 matching lines @@
 
 .network-timing-bar.proxy,
 .network-timing-bar.dns,
 .network-timing-bar.ssl,
 .network-timing-bar.connecting,
 .network-timing-bar.blocking {
     height: 10px;
     margin: auto;
 }
 
-.resource-timing-view.visible {
-    display: block;
-}
-
 .resource-timing-view .network-timing-table {
     width: 100%;
 }
 
 #network-overview-container {
     overflow: hidden;
     flex: auto;
     display: flex;
     position: relative;
     border-bottom: 1px solid #CDCDCD;
@@ skipping 45 matching lines @@
 .request-view.html iframe {
     width: 100%;
     height: 100%;
     position: absolute;
 }
 
 .network-film-strip {
     border-bottom: solid 1px #cdcdcd;
     flex: none !important;
 }