Allow ObserverListThreadSafe to be used from sequenced tasks.

components/ukm/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/ukm_service.h"
 
 #include <memory>
 #include <string>
 #include <utility>
 
 #include "base/feature_list.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/rand_util.h"
 #include "base/strings/string_number_conversions.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
 #include "components/metrics/metrics_log.h"
 #include "components/metrics/metrics_log_uploader.h"
 #include "components/metrics/metrics_service_client.h"
 #include "components/metrics/proto/ukm/report.pb.h"
 #include "components/metrics/proto/ukm/source.pb.h"
 #include "components/prefs/pref_registry_simple.h"
 #include "components/prefs/pref_service.h"
 #include "components/ukm/metrics_reporting_scheduler.h"
 #include "components/ukm/persisted_logs_metrics_impl.h"
@@ skipping 237 matching lines @@
 void UkmService::RecordSource(std::unique_ptr<UkmSource> source) {
   if (sources_.size() >= kMaxSources) {
     UMA_HISTOGRAM_BOOLEAN("UKM.Sources.MaxSourcesHit", true);
     return;
   }
 
   sources_.push_back(std::move(source));
 }
 
 }  // namespace ukm