Use StringPiece for CreateUploader/NetMetricsLogUploader params.

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 59fe49beffdc5c4ea18f83994e5d366064940d00..3253f6935b2a0888d3884c49b365e3b8aa428685 100644
--- a/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.h
+++ b/ios/chrome/browser/metrics/ios_chrome_metrics_service_client.h
@@ -72,8 +72,8 @@ class IOSChromeMetricsServiceClient
       const base::Closure& done_callback) override;
   void CollectFinalMetricsForLog(const base::Closure& done_callback) override;
   std::unique_ptr<metrics::MetricsLogUploader> CreateUploader(
-      const std::string& server_url,
-      const std::string& mime_type,
+      const base::StringPiece& server_url,
+      const base::StringPiece& mime_type,
       metrics::MetricsLogUploader::MetricServiceType service_type,
       const base::Callback<void(int)>& on_upload_complete) override;
   base::TimeDelta GetStandardUploadInterval() override;