Add UKM initial_url and session_id fields.

components/ukm/test_ukm_service.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/test_ukm_service.h"
 
 namespace ukm {
 
 UkmServiceTestingHarness::UkmServiceTestingHarness() {
   UkmService::RegisterPrefs(test_prefs_.registry());
   test_prefs_.ClearPref(prefs::kUkmClientId);
+  test_prefs_.ClearPref(prefs::kUkmSessionId);
   test_prefs_.ClearPref(prefs::kUkmPersistedLogs);
 
   test_ukm_service_ = base::MakeUnique<TestUkmService>(&test_prefs_);
 }
 
 UkmServiceTestingHarness::~UkmServiceTestingHarness() = default;
 
 TestUkmService::TestUkmService(PrefService* prefs_service)
     : UkmService(prefs_service, &test_metrics_service_client_) {
   EnableRecording();
   DisableReporting();
 }
 
 TestUkmService::~TestUkmService() = default;
 
 const UkmSource* TestUkmService::GetSource(size_t source_num) const {
   return sources_for_testing()[source_num].get();
 }
 
 const UkmEntry* TestUkmService::GetEntry(size_t entry_num) const {
   return entries_for_testing()[entry_num].get();
 }
 
 }  // namespace ukm