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

components/metrics/test_metrics_service_client.h

Index: components/metrics/test_metrics_service_client.h
diff --git a/components/metrics/test_metrics_service_client.h b/components/metrics/test_metrics_service_client.h
index 8710355901ce5486be15079275b49962130f12a3..bdb323c096dba227b575dab656e7546e6e928394 100644
--- a/components/metrics/test_metrics_service_client.h
+++ b/components/metrics/test_metrics_service_client.h
@@ -40,6 +40,8 @@ class TestMetricsServiceClient : public MetricsServiceClient {
   scoped_ptr<MetricsLogUploader> CreateUploader(
       const base::Callback<void(int)>& on_upload_complete) override;
   base::TimeDelta GetStandardUploadInterval() override;
+  scoped_refptr<base::SingleThreadTaskRunner> GetMainThreadTaskRunner()
+      override;
   bool IsReportingPolicyManaged() override;
   MetricsServiceClient::EnableMetricsDefault GetDefaultOptIn() override;
 
@@ -60,6 +62,7 @@ class TestMetricsServiceClient : public MetricsServiceClient {
   int32_t product_;
   bool reporting_is_managed_;
   MetricsServiceClient::EnableMetricsDefault enable_default_;
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(TestMetricsServiceClient);
 };