[EasyUnlock] Force user to enter their password after 20 hours.

components/proximity_auth/proximity_auth_pref_names.cc

 // 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.
 
 #include "components/proximity_auth/proximity_auth_pref_names.h"
 
 namespace proximity_auth {
 namespace prefs {
 
+// The timestamp of the last password entry in milliseconds, used to enforce
+// reauthing with the password after a given time period has elapsed.
+const char kProximityAuthLastPasswordEntryTimestampMs[] =
+    "proximity_auth.last_password_entry_timestamp_ms";
+
 // The dictionary containing remote BLE devices.
 const char kProximityAuthRemoteBleDevices[] =
     "proximity_auth.remote_ble_devices";
 
 }  // namespace prefs
 }  // namespace proximity_auth