Use StringPiece for CreateUploader/NetMetricsLogUploader params.

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 8d77e52a2883bab92296086524c63ab9cb76504f..0bda3ece38562574981ce5f45d58a3905769c0cb 100644
--- a/components/metrics/test_metrics_service_client.h
+++ b/components/metrics/test_metrics_service_client.h
@@ -37,8 +37,8 @@ class TestMetricsServiceClient : public MetricsServiceClient {
       const base::Closure& done_callback) override;
   void CollectFinalMetricsForLog(const base::Closure& done_callback) override;
   std::unique_ptr<MetricsLogUploader> CreateUploader(
-      const std::string& server_url,
-      const std::string& mime_type,
+      const base::StringPiece& server_url,
+      const base::StringPiece& mime_type,
       MetricsLogUploader::MetricServiceType service_type,
       const base::Callback<void(int)>& on_upload_complete) override;
   base::TimeDelta GetStandardUploadInterval() override;