Add U2F request state machines

device/u2f/u2f_return_code.h

Index: device/u2f/u2f_return_code.h
diff --git a/device/u2f/u2f_return_code.h b/device/u2f/u2f_return_code.h
new file mode 100644
index 0000000000000000000000000000000000000000..4aa0b4284e73230357efcf7f347828c1ff4389f4
--- /dev/null
+++ b/device/u2f/u2f_return_code.h
@@ -0,0 +1,19 @@
+// 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.
+
+#ifndef DEVICE_U2F_U2F_RETURN_CODE_H_
+#define DEVICE_U2F_U2F_RETURN_CODE_H_
+
+namespace device {
+
+enum class U2fReturnCode : uint8_t {
+  SUCCESS,
+  FAILURE,
+  INVALID_PARAMS,
+  CONDITIONS_NOT_SATISFIED,
+};
+
+}  // namespace device
+
+#endif  // DEVICE_U2F_U2F_RETURN_CODE_H_