[Smart Lock] Add a setting to require close proximity in order for unlocking to work.

components/proximity_auth/switches.cc

+// Copyright 2014 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/switches.h"
+
+namespace proximity_auth {
+namespace switches {
+
+// Enables close proximity detection. This allows the user to set a setting to
+// require very close proximity between the remote device and the local device
+// in order to unlock the local device, which trades off convenience for
+// security.
+const char kEnableProximityDetection[] =
+    "enable-proximity-auth-proximity-detection";
+
+}  // namespace switches
+}  // namespace proximity_auth