Introduce ParallelDownloadJob.

content/browser/download/download_job_impl.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <content/browser/download/download_job_impl.h>
+#include "content/public/browser/web_contents.h"
+
+namespace content {
+
+DownloadJobImpl::DownloadJobImpl(
+    DownloadItemImpl* download_item,
+    std::unique_ptr<DownloadRequestHandleInterface> request_handle)
+    : DownloadJob(download_item), request_handle_(std::move(request_handle)) {}
+
+DownloadJobImpl::~DownloadJobImpl() = default;
+
+void DownloadJobImpl::Start() {
+  StartDownloadProgress();
+}
+
+void DownloadJobImpl::Cancel(bool user_cancel) {
+  if (request_handle_)
+    request_handle_->CancelRequest();
+}
+
+void DownloadJobImpl::Pause() {
+  DownloadJob::Pause();
+  if (request_handle_)
+    request_handle_->PauseRequest();
+}
+
+void DownloadJobImpl::Resume() {
+  if (request_handle_)
+    request_handle_->ResumeRequest();
+
+  DownloadJob::Resume();
+}
+
+WebContents* DownloadJobImpl::GetWebContents() const {
+  return request_handle_ ? request_handle_->GetWebContents() : nullptr;
+}
+
+}  // namespace content