Use StringPiece for CreateUploader/NetMetricsLogUploader params.

ios/chrome/browser/metrics/ios_chrome_metrics_service_client.mm

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/chrome/browser/metrics/ios_chrome_metrics_service_client.h"
 
 #include <stdint.h>
 
 #include <utility>
 #include <vector>
@@ skipping 145 matching lines @@
     // |FinishedReceivingProfilerData()| when the task completes.
     metrics::TrackingSynchronizer::FetchProfilerDataAsynchronously(
         weak_ptr_factory_.GetWeakPtr());
   } else {
     CollectFinalHistograms();
   }
 }
 
 std::unique_ptr<metrics::MetricsLogUploader>
 IOSChromeMetricsServiceClient::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) {
   return base::MakeUnique<metrics::NetMetricsLogUploader>(
       GetApplicationContext()->GetSystemURLRequestContext(), server_url,
       mime_type, service_type, on_upload_complete);
 }
 
 base::TimeDelta IOSChromeMetricsServiceClient::GetStandardUploadInterval() {
   return metrics::GetUploadInterval();
 }
@@ skipping 196 matching lines @@
     ukm_service_->Purge();
     ukm_service_->ResetClientId();
   }
   // Signal service manager to enable/disable UKM based on new state.
   UpdateRunningServices();
 }
 
 bool IOSChromeMetricsServiceClient::IsHistorySyncEnabledOnAllProfiles() {
   return SyncDisableObserver::IsHistorySyncEnabledOnAllProfiles();
 }