Add UKM initial_url and session_id fields.

components/ukm/ukm_pref_names.cc

 // Copyright 2017 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/ukm/ukm_pref_names.h"
 
 namespace ukm {
 namespace prefs {
 
 // A random uint64 value unique for each chrome install.
 const char kUkmClientId[] = "ukm.client_id";
 
 // Preference which stores serialized UKM logs to be uploaded.
 const char kUkmPersistedLogs[] = "ukm.persisted_logs";
 
+// Preference which stores the UKM session id.
+const char kUkmSessionId[] = "ukm.session_id";
+
 }  // namespace prefs
 }  // namespace ukm