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

ios/chrome/browser/metrics/ios_chrome_metrics_service_client.cc

Index: ios/chrome/browser/metrics/ios_chrome_metrics_service_client.cc
diff --git a/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.cc b/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.cc
index 7223da6884c2a39472100fc7dab5a28949860b09..f9921f5ada0307a48e49b6dadc6ef56ed75dda41 100644
--- a/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.cc
+++ b/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.cc
@@ -192,6 +192,11 @@ base::TimeDelta IOSChromeMetricsServiceClient::GetStandardUploadInterval() {
   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();
 }