The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f61d39f8a151e11949079994228823ca9720cba1

commit f61d39f8a151e11949079994228823ca9720cba1
Author: xingliu <xingliu@chromium.org>
Date: Fri Jan 27 22:46:46 2017

Introduce DownloadJob for DownloadItem refactoring.

1. DownloadJob is to delegate actual download logic from
DownloadItemImpl. Also it can be used
for parallel download feature, which is a DownloadJob manages
list of smaller tasks(pieces of parallel download segments).

2. DownloadJob base class provides an content internal interface.
The public interface of DownloadItem is intact.

3. Add unit test and mock structure for DownloadJob.

This CL only checks in the interface. Pulling out
DownloadItemImpl logic and subclass of DownloadJob will come
in later CL.

BUG=682350

Review-Url: https://codereview.chromium.org/2645133003
Cr-Commit-Position: refs/heads/master@{#446806}

[modify] https://crrev.com/f61d39f8a151e11949079994228823ca9720cba1/content/browser/BUILD.gn
[add] https://crrev.com/f61d39f8a151e11949079994228823ca9720cba1/content/browser/download/download_job.cc
[add] https://crrev.com/f61d39f8a151e11949079994228823ca9720cba1/content/browser/download/download_job.h
[add] https://crrev.com/f61d39f8a151e11949079994228823ca9720cba1/content/browser/download/download_job_unittest.cc
[add] https://crrev.com/f61d39f8a151e11949079994228823ca9720cba1/content/browser/download/mock_download_job.cc
[add] https://crrev.com/f61d39f8a151e11949079994228823ca9720cba1/content/browser/download/mock_download_job.h
[modify] https://crrev.com/f61d39f8a151e11949079994228823ca9720cba1/content/test/BUILD.gn