Add XStream to Chromium third_party.

third_party/xstream/BUILD.gn

Index: third_party/xstream/BUILD.gn
diff --git a/third_party/xstream/BUILD.gn b/third_party/xstream/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..ee07a1e26ffcc52a1532cda73d884f559fbd8576
--- /dev/null
+++ b/third_party/xstream/BUILD.gn
@@ -0,0 +1,10 @@
+# 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.
+
+import("//build/config/android/rules.gni")
+
+java_prebuilt("xstream_java") {
+  testonly = true
+  jar_path = "lib/xstream-1.4.8.jar"
+}