More user id tracking for mus:

components/mus/ws/user_id.h

Index: components/mus/ws/user_id.h
diff --git a/components/mus/ws/user_id.h b/components/mus/ws/user_id.h
new file mode 100644
index 0000000000000000000000000000000000000000..26c9bb30357887419cc21c157a73f4285b9b4f9f
--- /dev/null
+++ b/components/mus/ws/user_id.h
@@ -0,0 +1,18 @@
+// 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.
+
+#ifndef COMPONENTS_MUS_WS_USER_ID_H_
+#define COMPONENTS_MUS_WS_USER_ID_H_
+
+#include <stdint.h>
+
+namespace mus {
+namespace ws {
+
+using UserId = uint32_t;
+
+}  // namespace ws
+}  // namespace mus
+
+#endif  // COMPONENTS_MUS_WS_USER_ID_H_