Introduce ParallelDownloadJob.

content/browser/BUILD.gn

Index: content/browser/BUILD.gn
diff --git a/content/browser/BUILD.gn b/content/browser/BUILD.gn
index 0a0561cbb9179f1c9fdfd2d486c5513256388b91..e4e0b73562fe28bc898f24d587525acaeaa7551a 100644
--- a/content/browser/BUILD.gn
+++ b/content/browser/BUILD.gn
@@ -556,6 +556,10 @@ 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",
@@ -568,12 +572,16 @@ 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",