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

Source/devtools/front_end/sources/uiList.css

 /*
  * Copyright 2015 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 .list-item {
-    padding: 3px 8px 4px 18px;
+    padding: 3px 8px 4px 8px;
     position: relative;
     min-height: 18px;
     white-space: nowrap;
 }
 
-.list-item:nth-of-type(2n) {
-    background-color: rgb(234, 243, 255);
+.list-item:hover {
+    background-color: #eee;
 }
 
-.list-item.selected {
-    background-color: #cfcfcf;
-    background-clip: padding-box;
+.list-item > .title::before {
+    content: "✓ ";
+    color: transparent;
+}
+
+.list-item.selected > .title::before {
+    color: inherit;
 }
 
 .list-item > .title {
     font-weight: normal;
     word-wrap: break-word;
     white-space: normal;
 }
 
+.list-item.selected > .title {
+    font-weight: bold;
+}
+
 .list-item > .subtitle {
     margin-left: 5px;
     color: rgba(0, 0, 0, 0.7);
     text-overflow: ellipsis;
     overflow: hidden;
     float: right;
 }
 
 .list-item > .subtitle a {
     color: inherit;
 }
 
 .list-item.label {
     text-align: center;
 }
 
 .list-item.label .title,
 .list-item.label .subtitle {
     font-style: italic;
     font-weight: bold;
     color: #999;
 }
 
 .list-item.dimmed {
     opacity: 0.6;
     font-style: italic;
 }