Add UKM initial_url and session_id fields.

components/ukm/ukm_pref_names.cc

Index: components/ukm/ukm_pref_names.cc
diff --git a/components/ukm/ukm_pref_names.cc b/components/ukm/ukm_pref_names.cc
index 75b374ff074ab4b92903398e23891b79f9fd4ec6..23ad6371a5d44288341f9fdb1888a11142b137c6 100644
--- a/components/ukm/ukm_pref_names.cc
+++ b/components/ukm/ukm_pref_names.cc
@@ -13,5 +13,8 @@ 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