Introduce DownloadJob for DownloadItem refactoring.

content/browser/download/mock_download_job.cc

Index: content/browser/download/mock_download_job.cc
diff --git a/content/browser/download/mock_download_job.cc b/content/browser/download/mock_download_job.cc
new file mode 100644
index 0000000000000000000000000000000000000000..9108e61701cd4e17b2159a70648df4c7c768aae4
--- /dev/null
+++ b/content/browser/download/mock_download_job.cc
@@ -0,0 +1,13 @@
+// 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/mock_download_job.h"
+
+namespace content {
+
+MockDownloadJob::MockDownloadJob() = default;
+
+MockDownloadJob::~MockDownloadJob() = default;
+
+}  // namespace content