Move the thread hop for UMA user actions from content:: to base::.

ios/chrome/browser/ios_chrome_main_parts.mm

 // Copyright 2016 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 "ios/chrome/browser/ios_chrome_main_parts.h"
 
 #include "base/base_switches.h"
 #include "base/feature_list.h"
 #include "base/files/file_path.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
+#include "base/metrics/user_metrics.h"
 #include "base/path_service.h"
 #include "base/time/default_tick_clock.h"
 #include "components/content_settings/core/browser/cookie_settings.h"
 #include "components/content_settings/core/common/content_settings_pattern.h"
 #include "components/flags_ui/pref_service_flags_storage.h"
 #include "components/language_usage_metrics/language_usage_metrics.h"
 #include "components/metrics/metrics_service.h"
 #include "components/metrics/profiler/ios/ios_tracking_synchronizer_delegate.h"
 #include "components/metrics/profiler/tracking_synchronizer.h"
 #include "components/metrics_services_manager/metrics_services_manager.h"
@@ skipping 160 matching lines @@
   TranslateServiceIOS::Shutdown();
   application_context_->StartTearDown();
 }
 
 void IOSChromeMainParts::PostDestroyThreads() {
   application_context_->PostDestroyThreads();
 }
 
 // This will be called after the command-line has been mutated by about:flags
 void IOSChromeMainParts::SetUpMetricsAndFieldTrials() {
+  base::SetRecordActionTaskRunner(

[Alexei Svitkine (slow), 2016/04/22 14:30:13]

Nit: Add a comment about where this is done on other platforms.

+      web::WebThread::GetTaskRunnerForThread(web::WebThread::UI));
+
   // Must initialize metrics after labs have been converted into switches,
   // but before field trials are set up (so that client ID is available for
   // one-time randomized field trials).
   metrics::MetricsService* metrics = application_context_->GetMetricsService();
 
   // Initialize FieldTrialList to support FieldTrials that use one-time
   // randomization.
   field_trial_list_.reset(
       new base::FieldTrialList(metrics->CreateEntropyProvider().release()));
 
@@ skipping 41 matching lines @@
 
 void IOSChromeMainParts::StartMetricsRecording() {
   bool wifiOnly = local_state_->GetBoolean(prefs::kMetricsReportingWifiOnly);
   bool isConnectionCellular = net::NetworkChangeNotifier::IsConnectionCellular(
       net::NetworkChangeNotifier::GetConnectionType());
   bool mayUpload = !wifiOnly || !isConnectionCellular;
 
   application_context_->GetMetricsServicesManager()->UpdateUploadPermissions(
       mayUpload);
 }