ChromeOS OOBE: Implement network selection screen "Back" button.

chrome/browser/resources/chromeos/login/oobe_back_button.css

+/* Copyright 2016 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 {
+  --paper-button-flat-keyboard-focus: {
+    border-color: rgba(66, 133, 244, 0.4); /* #4284f4 */
+    border-radius: 2px;
+    border-style: solid;
+    border-width: 2px;
+  };
+  color: var(--google-blue-500);
+  font: var(--oobe-roboto-bold), sans-serif;
+  font-size: 12px;
+  height: 32px; /* = 28 + border (2px + 2px) */
+  margin: 0;
+  min-width: 92px; /* = 88 + border (2px + 2px) */
+  padding: 0;
+}
+
+#button:not([focused]) {
+  border-color: transparent;
+  border-radius: 2px;
+  border-style: solid;
+  border-width: 2px;
+}
+
+#text {
+  -webkit-padding-end: 16px;
+  text-transform: none;
+}
+
+iron-icon {
+  --iron-icon-height: 12px;
+  --iron-icon-width: 12px;
+  -webkit-padding-end: 8px;
+  -webkit-padding-start: 16px;
+}
+
+#button[disabled] #text,
+#button[disabled] iron-icon {
+  color: rgba(0, 0, 0, 0.34);
+}
+