[+reilleyg]:  /3 looks to be the HEADERS_ONLY case.  

I'll defer to you, but may be best to revert https://chromium-review.googlesource.com/c/chromium/src/+/1161551 and fix it offline, though doesn't look like the flakiest test out there.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b624551f1c22ac5139c9ced011da1a15c2027b7c

commit b624551f1c22ac5139c9ced011da1a15c2027b7c
Author: Reilly Grant <reillyg@chromium.org>
Date: Tue Aug 07 23:40:05 2018

Revert "Port policy::UploadJobImpl to network::SimpleURLLoader"

This reverts commit 581330a9f8fa0cd9b3b83def1611740a186a4881.

Reason for revert: SimpleURLLoaderTest.OnUploadProgressCallback/3 is flaky

Original change's description:
> Port policy::UploadJobImpl to network::SimpleURLLoader
> 
> This change ports UploadJobImpl from net::URLFetcher to SimpleURLLoader.
> 
> Since this request does not make use of the response a new method has
> been added to SimpleURLLoader, DownloadToNull. This download mode still
> reads the response body from the network but does not save it anywhere.
> This is useful for requests which upload data and would otherwise have
> to specify an arbitrary |max_body_size|. A maximum may still be set if
> the caller wants to limit the amount of data transferred over the
> network.
> 
> Bug: 773295
> Cq-Include-Trybots: luci.chromium.try:linux_mojo
> Change-Id: Iee1fdc6f7406066ced8c91122e22cd51ddcb1c5f
> Reviewed-on: https://chromium-review.googlesource.com/1161551
> Commit-Queue: Reilly Grant <reillyg@chromium.org>
> Reviewed-by: Matt Menke <mmenke@chromium.org>
> Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#581060}

TBR=pastarmovj@chromium.org,reillyg@chromium.org,mmenke@chromium.org

Change-Id: Ica09c85a12861d35efbdfd9a882d137bac6b9e56
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 773295,  872023 
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Reviewed-on: https://chromium-review.googlesource.com/1166288
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581389}
[modify] https://crrev.com/b624551f1c22ac5139c9ced011da1a15c2027b7c/chrome/browser/chromeos/policy/remote_commands/screenshot_delegate.cc
[modify] https://crrev.com/b624551f1c22ac5139c9ced011da1a15c2027b7c/chrome/browser/chromeos/policy/system_log_uploader.cc
[modify] https://crrev.com/b624551f1c22ac5139c9ced011da1a15c2027b7c/chrome/browser/chromeos/policy/upload_job_impl.cc
[modify] https://crrev.com/b624551f1c22ac5139c9ced011da1a15c2027b7c/chrome/browser/chromeos/policy/upload_job_impl.h
[modify] https://crrev.com/b624551f1c22ac5139c9ced011da1a15c2027b7c/chrome/browser/chromeos/policy/upload_job_unittest.cc
[modify] https://crrev.com/b624551f1c22ac5139c9ced011da1a15c2027b7c/services/network/public/cpp/simple_url_loader.cc
[modify] https://crrev.com/b624551f1c22ac5139c9ced011da1a15c2027b7c/services/network/public/cpp/simple_url_loader.h
[modify] https://crrev.com/b624551f1c22ac5139c9ced011da1a15c2027b7c/services/network/public/cpp/simple_url_loader_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b4bc1f55ef9176428c0a0673b9574b6575f733d3

commit b4bc1f55ef9176428c0a0673b9574b6575f733d3
Author: Reilly Grant <reillyg@chromium.org>
Date: Tue Aug 07 23:56:28 2018

Reland "Port policy::UploadJobImpl to network::SimpleURLLoader"

This reverts commit b624551f1c22ac5139c9ced011da1a15c2027b7c.

Reason for revert: Reverting this change broke the tree because of a patch that depends on it. Since the test is only flaky fixing the tree is better than causing more disturbance by reverting more changes.

Original change's description:
> Revert "Port policy::UploadJobImpl to network::SimpleURLLoader"
> 
> This reverts commit 581330a9f8fa0cd9b3b83def1611740a186a4881.
> 
> Reason for revert: SimpleURLLoaderTest.OnUploadProgressCallback/3 is flaky
> 
> Original change's description:
> > Port policy::UploadJobImpl to network::SimpleURLLoader
> > 
> > This change ports UploadJobImpl from net::URLFetcher to SimpleURLLoader.
> > 
> > Since this request does not make use of the response a new method has
> > been added to SimpleURLLoader, DownloadToNull. This download mode still
> > reads the response body from the network but does not save it anywhere.
> > This is useful for requests which upload data and would otherwise have
> > to specify an arbitrary |max_body_size|. A maximum may still be set if
> > the caller wants to limit the amount of data transferred over the
> > network.
> > 
> > Bug: 773295
> > Cq-Include-Trybots: luci.chromium.try:linux_mojo
> > Change-Id: Iee1fdc6f7406066ced8c91122e22cd51ddcb1c5f
> > Reviewed-on: https://chromium-review.googlesource.com/1161551
> > Commit-Queue: Reilly Grant <reillyg@chromium.org>
> > Reviewed-by: Matt Menke <mmenke@chromium.org>
> > Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#581060}
> 
> TBR=pastarmovj@chromium.org,reillyg@chromium.org,mmenke@chromium.org
> 
> Change-Id: Ica09c85a12861d35efbdfd9a882d137bac6b9e56
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 773295,  872023 
> Cq-Include-Trybots: luci.chromium.try:linux_mojo
> Reviewed-on: https://chromium-review.googlesource.com/1166288
> Reviewed-by: Reilly Grant <reillyg@chromium.org>
> Commit-Queue: Reilly Grant <reillyg@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#581389}

TBR=pastarmovj@chromium.org,reillyg@chromium.org,mmenke@chromium.org

Change-Id: I793ab6c8c22c5bec97ef853f520dc506809970cb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 773295,  872023 
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Reviewed-on: https://chromium-review.googlesource.com/1166301
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581394}
[modify] https://crrev.com/b4bc1f55ef9176428c0a0673b9574b6575f733d3/chrome/browser/chromeos/policy/remote_commands/screenshot_delegate.cc
[modify] https://crrev.com/b4bc1f55ef9176428c0a0673b9574b6575f733d3/chrome/browser/chromeos/policy/system_log_uploader.cc
[modify] https://crrev.com/b4bc1f55ef9176428c0a0673b9574b6575f733d3/chrome/browser/chromeos/policy/upload_job_impl.cc
[modify] https://crrev.com/b4bc1f55ef9176428c0a0673b9574b6575f733d3/chrome/browser/chromeos/policy/upload_job_impl.h
[modify] https://crrev.com/b4bc1f55ef9176428c0a0673b9574b6575f733d3/chrome/browser/chromeos/policy/upload_job_unittest.cc
[modify] https://crrev.com/b4bc1f55ef9176428c0a0673b9574b6575f733d3/services/network/public/cpp/simple_url_loader.cc
[modify] https://crrev.com/b4bc1f55ef9176428c0a0673b9574b6575f733d3/services/network/public/cpp/simple_url_loader.h
[modify] https://crrev.com/b4bc1f55ef9176428c0a0673b9574b6575f733d3/services/network/public/cpp/simple_url_loader_unittest.cc

I'm going to leave this test flaky and fix it soon because trying to back out both my change conflicted with https://chromium-review.googlesource.com/c/chromium/src/+/1154971.

This isn't a flaky. The test fails because the patch (at patchset 8) doesn't check whether the download is in HEADERS_ONLY mode. This was fixed in the version that was landed (patchset 10).