Add UKM initial_url and session_id fields.

components/metrics/proto/ukm/report.proto

 // 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.
 
 syntax = "proto2";
 
 package ukm;
 
 option optimize_for = LITE_RUNTIME;
 
 import "ukm/entry.proto";
 import "ukm/source.proto";
 import "system_profile.proto";
 
 // This is the message type sent from Chrome to the UKM collector.
-// Next tag: 5
+// Next tag: 6
 message Report {
   // A unique identifier for a Chrome install. This ID should be used only
   // in UKM reports, and not linked to any other data sources.
   optional fixed64 client_id = 1;
 
+  // The session id for this record. This id is unique within a
+  // particular Chrome session. The client keeps track of the session id
+  // and sends it with each record. The session id is simply an integer
+  // that is incremented each time the user relaunches Chrome.
+  optional int32 session_id = 5;
+
   // Information about the user's browser and system configuration.
   optional metrics.SystemProfileProto system_profile = 2;
 
   // A list of the top-level navigations that data was collected for.
   repeated Source sources = 3;
 
   // List of Entries containing the main UKM data.
   repeated Entry entries = 4;
 }