Adding pref to store the user-selected proximity threshold.

components/proximity_auth/proximity_auth_pref_names.cc

Index: components/proximity_auth/proximity_auth_pref_names.cc
diff --git a/components/proximity_auth/proximity_auth_pref_names.cc b/components/proximity_auth/proximity_auth_pref_names.cc
index 3e0267e687de19f6d421c780f9785f049430fbb1..9ef234910c07ff316fbbfbfdb8e8831b8f08389f 100644
--- a/components/proximity_auth/proximity_auth_pref_names.cc
+++ b/components/proximity_auth/proximity_auth_pref_names.cc
@@ -16,5 +16,9 @@ const char kProximityAuthLastPasswordEntryTimestampMs[] =
 const char kProximityAuthRemoteBleDevices[] =
     "proximity_auth.remote_ble_devices";
 
+// How close the remote and the local device should be in order to use Easy
+// Unlock.
+const char kEasyUnlockProximityThreshold[] = "easy_unlock.proximity_threshold";
+
 }  // namespace prefs
 }  // namespace proximity_auth