I found the bug. It's a regression caused by me at https://chromium-review.googlesource.com/c/chromium/src/+/609683. In particular this change in this file https://chromium-review.googlesource.com/c/chromium/src/+/609683/17/components/feedback/feedback_report.cc#87 causes pending reports to be deleted on disk at shutdown.

This affects only reports made while offline and remained pending until sign out / re-sign in.

Each pending report upload is retired every 60, 120, 240, ... etc. minutes. If retry happens before sign out and device is online, report should be sent successfully.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/225aa5c0bd246043d112b6bd4fa1e9cf8f8237a1

commit 225aa5c0bd246043d112b6bd4fa1e9cf8f8237a1
Author: Ahmed Fakhry <afakhry@google.com>
Date: Thu Jun 14 03:22:02 2018

Feedback: Fix regression in sending offline reports.

Don't delete feedback reports files on disk on the
report destruction. This leads to losing pending reports
on sign out or reboot. Instead explicitly delete the files
once successfully uploaded, or when a no-retry failure occurs.

BUG= 851687 
TEST=Added new test.

Change-Id: I64caf16f63ea73c1888d8b097543e9eacabc1d9e
Reviewed-on: https://chromium-review.googlesource.com/1099758
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567111}
[modify] https://crrev.com/225aa5c0bd246043d112b6bd4fa1e9cf8f8237a1/components/feedback/feedback_report.cc
[modify] https://crrev.com/225aa5c0bd246043d112b6bd4fa1e9cf8f8237a1/components/feedback/feedback_uploader.cc
[modify] https://crrev.com/225aa5c0bd246043d112b6bd4fa1e9cf8f8237a1/components/feedback/feedback_uploader_unittest.cc

We are too late to hold the beta on this, but feel free to merge to 68 so we get this into the next release. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d7377b115ccb7816eca2b74995e8dd56f39b8149

commit d7377b115ccb7816eca2b74995e8dd56f39b8149
Author: Ahmed Fakhry <afakhry@google.com>
Date: Thu Jun 14 17:19:23 2018

[Merge to M-68] Feedback: Fix regression in sending offline reports.

Don't delete feedback reports files on disk on the
report destruction. This leads to losing pending reports
on sign out or reboot. Instead explicitly delete the files
once successfully uploaded, or when a no-retry failure occurs.

TBR=rkc@chromium.org
BUG= 851687 
TEST=Added new test.

(cherry picked from commit 225aa5c0bd246043d112b6bd4fa1e9cf8f8237a1)

Change-Id: I64caf16f63ea73c1888d8b097543e9eacabc1d9e
Reviewed-on: https://chromium-review.googlesource.com/1099758
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#567111}
Reviewed-on: https://chromium-review.googlesource.com/1101307
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/branch-heads/3440@{#361}
Cr-Branched-From: 010ddcfda246975d194964ccf20038ebbdec6084-refs/heads/master@{#561733}
[modify] https://crrev.com/d7377b115ccb7816eca2b74995e8dd56f39b8149/components/feedback/feedback_report.cc
[modify] https://crrev.com/d7377b115ccb7816eca2b74995e8dd56f39b8149/components/feedback/feedback_uploader.cc
[modify] https://crrev.com/d7377b115ccb7816eca2b74995e8dd56f39b8149/components/feedback/feedback_uploader_unittest.cc

Should we merge this to M-67 as well?

Having a gap in our feedback reports is pretty critical. If you think the change is safe, I'd lean towards merging to 67.

M67 if going into the last stable.  Is this a safe merge, and tested on M68 for verification?

Yes, it's a safe merge. I verified it on M-68 by sending a couple of feedback reports while offline, sign out, connect to network, and resign in. As expected the reports showed up on the server shortly: https://listnr.corp.google.com/product/208/reports?searchText=ovaf%20offline%20report%20test&filter=0&dateRange=All

Approving merge to M67 Chrome OS.

Thanks for the fast reply.  Please merge today if possible so it can make the next build.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cb31b673662c878c7dbf2c0458928543277c8392

commit cb31b673662c878c7dbf2c0458928543277c8392
Author: Ahmed Fakhry <afakhry@google.com>
Date: Thu Jun 21 19:08:34 2018

[Merge to M-67] Feedback: Fix regression in sending offline reports.

Don't delete feedback reports files on disk on the
report destruction. This leads to losing pending reports
on sign out or reboot. Instead explicitly delete the files
once successfully uploaded, or when a no-retry failure occurs.

TBR=rkc@chromium.org
BUG= 851687 
TEST=Added new test.

(cherry picked from commit 225aa5c0bd246043d112b6bd4fa1e9cf8f8237a1)

Change-Id: I64caf16f63ea73c1888d8b097543e9eacabc1d9e
Reviewed-on: https://chromium-review.googlesource.com/1099758
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#567111}
Reviewed-on: https://chromium-review.googlesource.com/1110526
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/branch-heads/3396@{#784}
Cr-Branched-From: 9ef2aa869bc7bc0c089e255d698cca6e47d6b038-refs/heads/master@{#550428}
[modify] https://crrev.com/cb31b673662c878c7dbf2c0458928543277c8392/components/feedback/feedback_report.cc
[modify] https://crrev.com/cb31b673662c878c7dbf2c0458928543277c8392/components/feedback/feedback_uploader.cc
[modify] https://crrev.com/cb31b673662c878c7dbf2c0458928543277c8392/components/feedback/feedback_uploader_unittest.cc