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

ios/chrome/browser/metrics/ios_chrome_metrics_service_client.h

Index: ios/chrome/browser/metrics/ios_chrome_metrics_service_client.h
diff --git a/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.h b/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.h
index f0014a37532f004f69205efed2b958f6c0f5305e..629b2bb9d132e8518d336b8fc44ea7d8bcae7d32 100644
--- a/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.h
+++ b/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.h
@@ -68,6 +68,8 @@ class IOSChromeMetricsServiceClient
   std::unique_ptr<metrics::MetricsLogUploader> CreateUploader(
       const base::Callback<void(int)>& on_upload_complete) override;
   base::TimeDelta GetStandardUploadInterval() override;
+  scoped_refptr<base::SingleThreadTaskRunner> GetMainThreadTaskRunner()
+      override;
   base::string16 GetRegistryBackupKey() override;
   void OnRendererProcessCrash() override;