arc: Implement scrollable OptIn window content.

chrome/browser/resources/chromeos/arc_support/lso.css

Index: chrome/browser/resources/chromeos/arc_support/lso.css
diff --git a/chrome/browser/resources/chromeos/arc_support/lso.css b/chrome/browser/resources/chromeos/arc_support/lso.css
new file mode 100644
index 0000000000000000000000000000000000000000..3e5556544960aebe58e5a108bf19c67a2873a092
--- /dev/null
+++ b/chrome/browser/resources/chromeos/arc_support/lso.css
@@ -0,0 +1,20 @@
+/**
+ * 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.
+ */
+
+::-webkit-scrollbar {
+  background: transparent;
+  height: 4px;
+  width: 4px;
+}
+
+::-webkit-scrollbar-thumb {
+  background: rgba(0, 0, 0, 0.3);
+  height: 16px;
+}
+
+::-webkit-scrollbar-thumb:hover {
+  background: rgba(0, 0, 0, 0.2);
+}