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

Source/devtools/front_end/sources/sourcesPanel.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 26 matching lines @@
 }
 
 .material .scripts-debug-toolbar {
     height: 28px;
     overflow: hidden;
 }
 
 .scripts-debug-toolbar {
     margin-top: -1px;
     height: 24px;
+    overflow: hidden;
 }
 
 .scripts-debug-toolbar-drawer {
     flex: 0 0 46px;
     -webkit-transition: margin-top 0.1s ease-in-out;
     margin-top: -23px;
     padding-top: 22px;
     border-bottom: 1px solid rgb(202, 202, 202);
     background-color: white;
     overflow: hidden;
@@ skipping 106 matching lines @@
     position: absolute;
     right: 3px;
     display: none;
 }
 
 .watch-expression-header:hover .watch-expression-delete-button {
     display: inline;
 }
 
 .watch-expressions {
-    margin-top: 14px;
     overflow-x: hidden;
     padding: 3px 6px 6px 0px;
 }
 
 .watch-expressions .dimmed {
     opacity: 0.6;
 }
 
 .watch-expression-title {
     white-space: nowrap;
@@ skipping 74 matching lines @@
 
 .scope-chain-sidebar-pane-section-title {
     font-weight: normal;
     word-wrap: break-word;
     white-space: normal;
 }
 
 .scope-chain-sidebar-pane-section {
     padding: 2px 4px 2px 4px;
 }