Fix issues and feature polishing for parallel download.

content/browser/download/parallel_download_job.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_JOB_H_
 #define CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_JOB_H_
 
 #include <memory>
 #include <unordered_map>
 #include <vector>
@@ skipping 21 matching lines @@
   // DownloadJobImpl implementation.
   void Start() override;
   void Cancel(bool user_cancel) override;
   void Pause() override;
   void Resume(bool resume_request) override;
   bool UsesParallelRequests() const override;
 
  protected:
   // Virtual for testing.
   virtual int GetParallelRequestCount() const;
+  virtual int64_t GetMinSliceSize() const;
 
   using WorkerMap =
       std::unordered_map<int64_t, std::unique_ptr<DownloadWorker>>;
 
   // Map from the offset position of the slice to the worker that downloads the
   // slice.
   WorkerMap workers_;
 
  private:
   friend class ParallelDownloadJobTest;
@@ skipping 15 matching lines @@
 
   // Build one http request for each slice from the second slice.
   // The first slice represents the original request.
   void ForkSubRequests(const DownloadItem::ReceivedSlices& slices_to_download);
 
   // Create one range request, virtual for testing.
   virtual void CreateRequest(int64_t offset, int64_t length);
 
   // Information about the initial request when download is started.
   int64_t initial_request_offset_;
+  int64_t initial_request_length_;
 
-  // The length of the response body of the original request. May be less than
-  // the size of the target file if the request starts from non-zero offset.
+  // The length of the response body of the original request.
+  // Used to estimate the remaining size of the content when the initial
+  // request is half open, i.e, |initial_request_length_| is
+  // DownloadSaveInfo::kLengthFullContent.
   int64_t content_length_;
 
   // Used to send parallel requests after a delay based on Finch config.
   base::OneShotTimer timer_;
 
   // If we have sent parallel requests.
   bool requests_sent_;
 
   // If the download progress is canceled.
   bool is_canceled_;
 
   DISALLOW_COPY_AND_ASSIGN(ParallelDownloadJob);
 };
 
 }  //  namespace content
 
 #endif  // CONTENT_BROWSER_DOWNLOAD_PARALLEL_DOWNLOAD_JOB_H_