Introduce DownloadJob for DownloadItem refactoring.

content/browser/download/download_job.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.h"
+
+namespace content {
+
+// Unknown download progress.
+const int kDownloadProgressUnknown = -1;
+
+// Unknown download speed.
+const int kDownloadSpeedUnknown = -1;
+
+DownloadJob::DownloadJob() : manager_(nullptr) {}
+
+DownloadJob::~DownloadJob() = default;
+
+void DownloadJob::OnAttached(DownloadJob::Manager* manager) {
+  DCHECK(!manager_) << "DownloadJob::Manager has already been attached.";
+  manager_ = manager;
+}
+
+void DownloadJob::OnBeforeDetach() {
+  manager_ = nullptr;
+}
+
+void DownloadJob::StartedSavingResponse() {
+  if (manager_)
+    manager_->OnSavingStarted(this);
+}
+
+void DownloadJob::Interrupt(DownloadInterruptReason reason) {
+  if (manager_)
+    manager_->OnDownloadInterrupted(this, reason);
+}
+
+void DownloadJob::Complete() {
+  if (manager_)
+    manager_->OnDownloadComplete(this);
+}
+
+}  // namespace content