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

third_party/WebKit/Source/devtools/front_end/ui/inspectorCommon.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.
  */
 
 * {
     /* This is required for correct sizing of flex items because we rely
      * on an old version of the flexbox spec.
      * Longer-term we should remove this, see crbug.com/473625 */
@@ skipping 54 matching lines @@
     left: 0;
     right: 0;
     bottom: 0;
 }
 
 iframe.fill {
     width: 100%;
     height: 100%;
 }
 
+iframe.extension {
+    flex: auto;
+    width: 100%;
+    height: 100%;
+}
+
+iframe.panel.extension {
+    display: block;
+    height: 100%;
+}
+
 .widget {
     position: relative;
     flex: auto;
 }
 
 .hbox {
     display: flex;
     flex-direction: row !important;
     position: relative;
 }
@@ skipping 222 matching lines @@
     padding: 20px;
     position: absolute;
     top: 0;
     right: 0;
     bottom: 0;
     left: 0;
     font-size: 13px;
     overflow: auto;
     z-index: 500;
 }