Use StringPiece for CreateUploader/NetMetricsLogUploader params.

Makes MetricsLogUploader a pure interface, eliminating the params there, and
in TestMetricsLogUploader.

TBR=boliu,olivierrobin

BUG=693676

Review-Url: https://codereview.chromium.org/2735443003
Cr-Commit-Position: refs/heads/master@{#455308}
diff --git a/chrome/browser/metrics/chrome_metrics_service_client.h b/chrome/browser/metrics/chrome_metrics_service_client.h
index dad283c..13ccabd 100644
--- a/chrome/browser/metrics/chrome_metrics_service_client.h
+++ b/chrome/browser/metrics/chrome_metrics_service_client.h
@@ -79,8 +79,8 @@
       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,
+      base::StringPiece server_url,
+      base::StringPiece mime_type,
       metrics::MetricsLogUploader::MetricServiceType service_type,
       const base::Callback<void(int)>& on_upload_complete) override;
   base::TimeDelta GetStandardUploadInterval() override;