Glue parallel download job and download file together.

content/browser/download/download_file_impl.h

Index: content/browser/download/download_file_impl.h
diff --git a/content/browser/download/download_file_impl.h b/content/browser/download/download_file_impl.h
index 61f1f4d90bb054a524b32e402f78111c7d1de5a0..f9101763f493153125da247f6f2f6180a2aba4ea 100644
--- a/content/browser/download/download_file_impl.h
+++ b/content/browser/download/download_file_impl.h
@@ -58,7 +58,8 @@ class CONTENT_EXPORT DownloadFileImpl : public DownloadFile {
   void Initialize(const InitializeCallback& callback) override;
 
   void AddByteStream(std::unique_ptr<ByteStreamReader> stream_reader,
-                     int64_t offset) override;
+                     int64_t offset,
+                     int64_t length) override;
 
   void RenameAndUniquify(const base::FilePath& full_path,
                          const RenameCompletionCallback& callback) override;
@@ -94,8 +95,6 @@ class CONTENT_EXPORT DownloadFileImpl : public DownloadFile {
   // |stream_reader_| can be set later when the network response is handled.
   //
   // Multiple SourceStreams can concurrently write to the same file sink.
-  //
-  // The file IO processing is finished when all SourceStreams are finished.
   class CONTENT_EXPORT SourceStream {
    public:
     SourceStream(int64_t offset, int64_t length);
@@ -203,11 +202,8 @@ class CONTENT_EXPORT DownloadFileImpl : public DownloadFile {
 
   // Map of the offset and the source stream that represents the slice
   // starting from offset.
-  // Must be created on the same thread that constructs the DownloadFile.
-  // Should not add or remove elements after creation.
-  // Any byte stream should have a SourceStream before added to the download
-  // file.
-  // The disk IO is completed when all source streams are finished.
+  // Can be modified in constructor on any thread, later must be accessed on
+  // file thread.

[asanka, 2017/03/14 19:44:01]

Doesn't need to be restated here. The thread restriction applies to all
DownloadFileImpl members.

[xingliu, 2017/03/14 22:48:29]

Done.

   SourceStreams source_streams_;
 
   // Used to trigger progress updates.