Add a metric to record http response code for download requests.

content/browser/download/download_request_core.cc

Index: content/browser/download/download_request_core.cc
diff --git a/content/browser/download/download_request_core.cc b/content/browser/download/download_request_core.cc
index e5c28d806acd21ae93e7cde58106fa8f6537d7cb..fc7f2e497e3583281fc61a1ee07d121f03f7bf3e 100644
--- a/content/browser/download/download_request_core.cc
+++ b/content/browser/download/download_request_core.cc
@@ -251,6 +251,9 @@ bool DownloadRequestCore::OnResponseStarted(
                                            save_info_.get())
           : DOWNLOAD_INTERRUPT_REASON_NONE;
 
+  RecordDownloadHttpResponseCode(

[qinmin, 2017/05/04 22:30:58]

I think you should do this in OnResponseCompleted rather than here

[qinmin, 2017/05/04 22:47:31]

Never mind, SERVER_FAILED was only reported in HandleSuccessfulServerResponse,
but I think we will be more interested in all the interruption reasons, with the
majority coming from OnResponseCompleted().

+      request()->response_headers()->response_code());
+
   std::unique_ptr<DownloadCreateInfo> create_info =
       CreateDownloadCreateInfo(result);
   if (result != DOWNLOAD_INTERRUPT_REASON_NONE) {