arc: Add splash screen for ARC++ Kiosk startup

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

+/* Copyright 2017 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. */
+
+#arc-kiosk-splash {
+  -webkit-align-items: center;
+  -webkit-justify-content: center;
+  background: white;
+  display: flex;
+}
+
+#arc-splash-content {
+  -webkit-align-items: center;
+  -webkit-justify-content: center;
+  display: flex;
+  flex-direction: column;
+  position: relative;
+}
+
+#arc-splash-header {
+  -webkit-padding-start: 108px;
+  background: left center no-repeat;
+  background-size: 96px;
+  color: #666;
+  display: flex;
+  font-size: 48px;
+  height: 96px;
+  line-height: 96px;
+}
+
+#arc-splash-launch-text {
+  color: #666;
+  font-size: 18px;
+  padding-top: 50px;
+}
+
+#arc-splash-spinner {
+  height: 32px;
+  width: 32px;
+}
+
+#arc-splash-shortcut-info {
+  bottom: 50px;
+  color: gray;
+  left: 0;
+  position: absolute;
+  right: 0;
+  text-align: center;
+}