Add UMA to track why a download is non-parallel when enabled parallel

content/browser/download/parallel_download_utils.h

 // Copyright 2017 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.
 
 #ifndef CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_UTILS_H_
 #define CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_UTILS_H_
 
 #include <vector>
 
 #include "content/browser/download/download_create_info.h"
 #include "content/common/content_export.h"
 #include "content/public/browser/download_item.h"
 
 namespace content {
 
-// Return if the download should be a parallel download.
-CONTENT_EXPORT bool ShouldUseParallelDownload(
-    const DownloadCreateInfo& create_info);
-
 // Chunks the content that starts from |current_offset|, into at most
 // std::max(|request_count|, 1) smaller slices.
 // Each slice contains at least |min_slice_size| bytes unless |total_length|
 // is less than |min_slice_size|.
 // The last slice is half opened.
 CONTENT_EXPORT std::vector<DownloadItem::ReceivedSlice>
 FindSlicesForRemainingContent(int64_t current_offset,
                               int64_t total_length,
                               int request_count,
                               int64_t min_slice_size);
@@ skipping 24 matching lines @@
 // Get the time delay to send parallel requests after the response of original
 // request is handled.
 CONTENT_EXPORT base::TimeDelta GetParallelRequestDelayConfig();
 
 // Print the states of received slices for debugging.
 CONTENT_EXPORT void DebugSlicesInfo(const DownloadItem::ReceivedSlices& slices);
 
 }  //  namespace content
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_UTILS_H_