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

chrome/browser/resources/media_router/elements/issue_banner/issue_banner.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. */
 
-.button-secondary {
-  background: linear-gradient(to top, rgb(254, 254, 254), rgb(222, 222, 222));
-}
-
-.button-action {
-  background: linear-gradient(to top, rgb(89, 158, 244), rgb(75, 134, 241));
-  border: 1px solid rgb(55, 113, 206);
+.action {
+  /* TODO(apacible): Update to use Chromium paper color. */
+  background-color: rgb(82, 147, 242);
   color: white;
 }
 
-.issue-action {
-  color: black;
-  cursor: pointer;
-  text-decoration: underline;
-}
-
 .blocking-issue-header {
   display: flex;
   overflow: hidden;
   padding-top: 10px;
 }
 
 .blocking-issue-message {
   -webkit-padding-start: 8px;
   color: rgb(125, 125, 125);
   line-height: 1.125em;
   vertical-align: middle;
 }
 
 .blocking-issue-title {
   -webkit-padding-start: 8px;
   font-weight: bold;
   line-height: 1.125em;
   vertical-align: top;
 }
 
+.issue-action {
+  color: black;
+  cursor: pointer;
+  text-decoration: underline;
+}
+
+#issue-buttons {
+  display: flex;
+}
+
 .non-blocking-issue {
   -webkit-padding-start: 12px;
   background-color: rgb(249, 236, 191);
   line-height: 1.25em;
   padding-bottom: 6px;
   padding-top: 6px;
 }
 
 .sad-face {
   -webkit-margin-start: 20px;
   background-image: -webkit-image-set(
     url(../icon/sad-face.png) 1x,
     url(../icon/sad-face2x.png) 2x);
   background-repeat: no-repeat;
   height: 32px;
   width: 32px;
 }