Use a stable device-user identifier for CryptAuth enrollments.

components/proximity_auth/cryptauth/cryptauth_enrollment_utils.h

Index: components/proximity_auth/cryptauth/cryptauth_enrollment_utils.h
diff --git a/components/proximity_auth/cryptauth/cryptauth_enrollment_utils.h b/components/proximity_auth/cryptauth/cryptauth_enrollment_utils.h
new file mode 100644
index 0000000000000000000000000000000000000000..0bebd1a2b1123dc25cef21e7a5571627267b8795
--- /dev/null
+++ b/components/proximity_auth/cryptauth/cryptauth_enrollment_utils.h
@@ -0,0 +1,18 @@
+// Copyright 2015 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_PROXIMITY_AUTH_CRYPT_AUTH_ENROLLMENT_UTILS_H
+#define COMPONENTS_PROXIMITY_AUTH_CRYPT_AUTH_ENROLLMENT_UTILS_H
+
+#include <string>
+
+namespace proximity_auth {
+
+// Calculates an id for the specified user on the specified device.
+std::string CalculateDeviceUserId(const std::string& device_id,
+                                  const std::string& user_id);
+
+}  // namespace proximity_auth
+
+#endif  // COMPONENTS_PROXIMITY_AUTH_CRYPT_AUTH_ENROLLMENT_UTILS_H