Introduce ParallelDownloadJob.

content/browser/BUILD.gn

Index: content/browser/BUILD.gn
diff --git a/content/browser/BUILD.gn b/content/browser/BUILD.gn
index 5b5af15d0990a79997eede79d086c44931f5fa51..d4e9ff747e2a7b81daa0e9a444424ede001bbdb6 100644
--- a/content/browser/BUILD.gn
+++ b/content/browser/BUILD.gn
@@ -561,6 +561,10 @@ source_set("browser") {
     "download/download_item_impl_delegate.h",
     "download/download_job.cc",
     "download/download_job.h",
+    "download/download_job_factory.cc",
+    "download/download_job_factory.h",
+    "download/download_job_impl.cc",
+    "download/download_job_impl.h",
     "download/download_manager_impl.cc",
     "download/download_manager_impl.h",
     "download/download_net_log_parameters.cc",
@@ -573,12 +577,16 @@ source_set("browser") {
     "download/download_resource_handler.h",
     "download/download_stats.cc",
     "download/download_stats.h",
+    "download/download_worker.cc",
+    "download/download_worker.h",
     "download/drag_download_file.cc",
     "download/drag_download_file.h",
     "download/drag_download_util.cc",
     "download/drag_download_util.h",
     "download/mhtml_generation_manager.cc",
     "download/mhtml_generation_manager.h",
+    "download/parallel_download_job.cc",
+    "download/parallel_download_job.h",
     "download/rate_estimator.cc",
     "download/rate_estimator.h",
     "download/save_file.cc",