Glue parallel download job and download file together.

content/browser/download/download_file_impl.cc

Index: content/browser/download/download_file_impl.cc
diff --git a/content/browser/download/download_file_impl.cc b/content/browser/download/download_file_impl.cc
index e4ec806f7718c8b4de357eb9977f329a852834a7..88eb1f5ab3e8b68cdc345626197bab6c6a58d7e7 100644
--- a/content/browser/download/download_file_impl.cc
+++ b/content/browser/download/download_file_impl.cc
@@ -120,7 +120,7 @@ void DownloadFileImpl::Initialize(const InitializeCallback& callback) {
   // Primarily to make reset to zero in restart visible to owner.
   SendUpdate();
 
-  // Initial pull from the straw.
+  // Initial pull from the straw from all source streams.
   for (auto& source_stream : source_streams_)
     RegisterAndActivateStream(source_stream.second.get());
 
@@ -131,16 +131,18 @@ void DownloadFileImpl::Initialize(const InitializeCallback& callback) {
 
 void DownloadFileImpl::AddByteStream(
     std::unique_ptr<ByteStreamReader> stream_reader,
-    int64_t offset) {
+    int64_t offset,
+    int64_t length) {
   DCHECK_CURRENTLY_ON(BrowserThread::FILE);
+  DCHECK(source_streams_.find(offset) == source_streams_.end());
 
-  // The |source_streams_| must have an existing entry for the stream reader.
-  auto current_source_stream = source_streams_.find(offset);
-  DCHECK(current_source_stream != source_streams_.end());
-  SourceStream* stream = current_source_stream->second.get();
+  source_streams_[offset] = base::MakeUnique<SourceStream>(offset, length);
+  SourceStream* stream = source_streams_[offset].get();
   stream->SetByteStream(std::move(stream_reader));
 
-  RegisterAndActivateStream(stream);
+  // If the file is initialized, start to write data, or wait until file opened.
+  if (file_.in_progress())
+    RegisterAndActivateStream(stream);
 }
 
 DownloadInterruptReason DownloadFileImpl::WriteDataToFile(int64_t offset,