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

content/browser/download/download_request_core.cc

 // Copyright (c) 2012 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 "content/browser/download/download_request_core.h"
 
 #include <string>
 
 #include "base/bind.h"
 #include "base/callback_helpers.h"
@@ skipping 233 matching lines @@
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   DVLOG(20) << __func__ << "() " << DebugString();
   download_start_time_ = base::TimeTicks::Now();
 
   DownloadInterruptReason result =
       request()->response_headers()
           ? HandleSuccessfulServerResponse(*request()->response_headers(),
                                            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) {
     delegate_->OnStart(std::move(create_info),
                        std::unique_ptr<ByteStreamReader>(),
                        base::ResetAndReturn(&on_started_callback_));
     return false;
   }
 
   // If it's a download, we don't want to poison the cache with it.
@@ skipping 426 matching lines @@
   // old servers that didn't implement "If-Match" and must be ignored when
   // "If-Match" presents.
   if (has_last_modified) {
     request->SetExtraRequestHeaderByName(
         net::HttpRequestHeaders::kIfUnmodifiedSince, params->last_modified(),
         true);
   }
 }
 
 }  // namespace content