[Media Router WebUI] Handle header height changes.

chrome/browser/resources/media_router/media_router_common.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. */
 
 :root {
-  --container-header-height: 52px;
   --dialog-padding-end: 26px;
   --dialog-padding-start: 16px;
   --navigation-icon-size: 20px;
   --non-navigation-icon-size: 16px;
   --subtext-color: rgb(150, 150, 150);
 }
 
 .button {
   color: var(--paper-blue-500);
   cursor: pointer;
   text-align: center;
 }
 
 [hidden] {
   display: none !important;
-}
+}