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

Source/devtools/front_end/resources/resourcesPanel.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 85 matching lines @@
     line-height: 1em;
 }
 
 .resources.panel li .status .bubble-repeat-count {
     height: 13px;
     margin-top: 1px;
     padding-top: 0;
 }
 
 .storage-view {
-    display: none;
+    display: flex;
     overflow: hidden;
 }
 
-.storage-view.visible {
-    display: flex;
-}
-
 .storage-view {
     overflow: hidden;
 }
 
 .storage-view .data-grid:not(.inline) {
     border: none;
     flex: auto;
 }
 
 .storage-view .storage-table-error {
@@ skipping 226 matching lines @@
     content: url(Images/cookie.png);
 }
 
 .application-cache-storage-tree-item .icon {
     content: url(Images/applicationCache.png);
 }
 
 .file-system-storage-tree-item .icon {
     content: url(Images/fileSystem.png);
 }