Use StringPiece for CreateUploader/NetMetricsLogUploader params.

ios/chrome/today_extension/today_metrics_logger.mm

Index: ios/chrome/today_extension/today_metrics_logger.mm
diff --git a/ios/chrome/today_extension/today_metrics_logger.mm b/ios/chrome/today_extension/today_metrics_logger.mm
index 8022d431e17a80d7562de7719f8eda8da96677e8..cb234f5fb92800e926cfd7a072560e9cab1ea412 100644
--- a/ios/chrome/today_extension/today_metrics_logger.mm
+++ b/ios/chrome/today_extension/today_metrics_logger.mm
@@ -69,8 +69,8 @@ class TodayMetricsServiceClient : public metrics::MetricsServiceClient {
       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;
@@ -150,8 +150,8 @@ void TodayMetricsServiceClient::CollectFinalMetricsForLog(
 
 std::unique_ptr<metrics::MetricsLogUploader>
 TodayMetricsServiceClient::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) {
   NOTREACHED();