Introduce ParallelDownloadJob.

content/browser/download/download_manager_impl.h

Index: content/browser/download/download_manager_impl.h
diff --git a/content/browser/download/download_manager_impl.h b/content/browser/download/download_manager_impl.h
index 859fe17a774d60717c6da01a929d3e9a7bff845d..4ffe65183541cc9e5a72ff12db610f6590810d63 100644
--- a/content/browser/download/download_manager_impl.h
+++ b/content/browser/download/download_manager_impl.h
@@ -40,6 +40,7 @@ class DownloadRequestHandleInterface;
 class ResourceContext;
 
 class CONTENT_EXPORT DownloadManagerImpl : public DownloadManager,
+                                           public UrlDownloader::Delegate,
                                            private DownloadItemImplDelegate {
  public:
   using DownloadItemImplCreated = base::Callback<void(DownloadItemImpl*)>;
@@ -111,6 +112,13 @@ class CONTENT_EXPORT DownloadManagerImpl : public DownloadManager,
   DownloadItem* GetDownload(uint32_t id) override;
   DownloadItem* GetDownloadByGuid(const std::string& guid) override;
 
+  // UrlDownloader::Delegate implementation.
+  void OnUrlDownloaderStarted(
+      std::unique_ptr<DownloadCreateInfo> download_create_info,
+      std::unique_ptr<ByteStreamReader> stream_reader,
+      const DownloadUrlParameters::OnStartedCallback& callback) override;
+  void OnUrlDownloaderStopped(UrlDownloader* downloader) override;
+
   // For testing; specifically, accessed from TestFileErrorInjector.
   void SetDownloadItemFactoryForTesting(
       std::unique_ptr<DownloadItemFactory> item_factory);
@@ -118,8 +126,6 @@ class CONTENT_EXPORT DownloadManagerImpl : public DownloadManager,
       std::unique_ptr<DownloadFileFactory> file_factory);
   virtual DownloadFileFactory* GetDownloadFileFactoryForTesting();
 
-  void RemoveUrlDownloader(UrlDownloader* downloader);
-
   // Helper function to initiate a download request. This function initiates
   // the download using functionality provided by the
   // ResourceDispatcherHostImpl::BeginURLRequest function. The function returns