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

chrome/browser/resources/media_router/elements/issue_banner/issue_banner.css

Index: chrome/browser/resources/media_router/elements/issue_banner/issue_banner.css
diff --git a/chrome/browser/resources/media_router/elements/issue_banner/issue_banner.css b/chrome/browser/resources/media_router/elements/issue_banner/issue_banner.css
index 4a1ced01e34f5879fc078c7f718e75619b658180..04ecb39adf5df0ded3770f8e991e9ada698f8c85 100644
--- a/chrome/browser/resources/media_router/elements/issue_banner/issue_banner.css
+++ b/chrome/browser/resources/media_router/elements/issue_banner/issue_banner.css
@@ -2,22 +2,12 @@
  * 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;
@@ -38,6 +28,16 @@
   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);