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-box-align: center;
+  -webkit-box-pack: center;
+  background: white;
+  display: -webkit-box;
+}
+
+#arc-splash-content {
+  -webkit-box-align: center;
+  -webkit-box-orient: vertical;
+  -webkit-box-pack: center;
+  display: -webkit-box;
+  position: relative;
+}
+
+#arc-splash-header {
+  -webkit-padding-start: 108px;
+  background: left center no-repeat;
+  background-size: 96px;
+  color: #666;
+  display: -webkit-box;
+  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;
+}