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

components/proximity_auth/switches.cc

Index: components/proximity_auth/switches.cc
diff --git a/components/proximity_auth/switches.cc b/components/proximity_auth/switches.cc
new file mode 100644
index 0000000000000000000000000000000000000000..ba5aab7f52ba4a2486c9c675501b1c9345d39160
--- /dev/null
+++ b/components/proximity_auth/switches.cc
@@ -0,0 +1,18 @@
+// 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