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

ios/chrome/browser/metrics/ios_chrome_metrics_service_client.cc

 // Copyright 2015 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/metrics/ios_chrome_metrics_service_client.h"
 
 #include <stdint.h>
 
 #include <utility>
 #include <vector>
@@ skipping 174 matching lines @@
           metrics::kDefaultMetricsServerUrl, metrics::kDefaultMetricsMimeType,
           on_upload_complete));
 }
 
 base::TimeDelta IOSChromeMetricsServiceClient::GetStandardUploadInterval() {
   if (IsCellularLogicEnabled())
     return base::TimeDelta::FromSeconds(kStandardUploadIntervalCellularSeconds);
   return base::TimeDelta::FromSeconds(kStandardUploadIntervalSeconds);
 }
 
+scoped_refptr<base::SingleThreadTaskRunner>
+IOSChromeMetricsServiceClient::GetMainThreadTaskRunner() {
+  return web::WebThread::GetTaskRunnerForThread(web::WebThread::UI);
+}
+
 base::string16 IOSChromeMetricsServiceClient::GetRegistryBackupKey() {
   return base::string16();
 }
 
 void IOSChromeMetricsServiceClient::OnRendererProcessCrash() {
   stability_metrics_provider_->LogRendererCrash();
 }
 
 void IOSChromeMetricsServiceClient::WebStateDidStartLoading(
     web::WebState* web_state) {
@@ skipping 125 matching lines @@
                      base::Unretained(this)));
 }
 
 void IOSChromeMetricsServiceClient::OnTabParented(web::WebState* web_state) {
   metrics_service_->OnApplicationNotIdle();
 }
 
 void IOSChromeMetricsServiceClient::OnURLOpenedFromOmnibox(OmniboxLog* log) {
   metrics_service_->OnApplicationNotIdle();
 }