[DevTools] Turn WebInspector.Panel into a web component.

third_party/WebKit/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 72 matching lines @@
     display: flex;
     align-items: center;
     position: relative;
     margin-left: 10px;
 }
 
 .function-popover-title .function-name {
     font-weight: bold;
 }
 
-.panel.sources .sidebar-pane-stack {
+.panel .sidebar-pane-stack {
     overflow: auto;
 }
 
 .cursor-pointer {
     cursor: pointer;
 }
 
 .cursor-auto {
     cursor: auto;
 }
@@ skipping 161 matching lines @@
     text-align: center;
     font-style: italic;
     padding: 4px;
     color: #888;
     background-color: #FFFFC2;
 }
 
 .event-listeners-sidebar-pane .toolbar {
     border-bottom: 1px solid #eee;
 }