Convert Media Router WebUI <button>s to <paper-button>s.

chrome/browser/resources/media_router/elements/route_details/route_details.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. */
 
 #back-to-devices {
   -webkit-padding-start: 10px;
   color: rgb(20, 86, 218);
   font-weight: bold;
   line-height: 2em;
 }
 
 #back-to-devices:hover {
   cursor: pointer;
   text-decoration: underline;
 }
 
-button {
-  background: linear-gradient(to top, rgb(254, 254, 254), rgb(222, 222, 222));
-}
-
 hr {
   margin: 0;
 }
 
 .route {
   -webkit-padding-start: 10px;
   line-height: 1.125em;
 }
 
 .route-status {
   color: rgb(125, 125, 125);
   margin: 8px 0;
 }
 
 .route-title {
   font-weight: bold;
   margin: 8px 0;
   overflow: hidden;
 }