The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8901493d98ea26516d5e794e5afba6f45d204c3d

commit 8901493d98ea26516d5e794e5afba6f45d204c3d
Author: Kuo-Hsin Yang <vovoy@chromium.org>
Date: Fri Apr 13 13:00:30 2018

Fix Chrome OS tab discard regression

https://crrev.com/c/980742 replaces EstimatedMemoryFreedKB() with
GetEstimatedMemoryFreedOnDiscardKB() which always returns 0. It confuses
Chrome OS Tab Manager and all tabs except the front tab are discarded.
It's a quick fix to make tab discard work on Chrome OS.

Bug:  832517 
Change-Id: Ic72be7a7535bf6e0a48a6f6efa9afc28635813c8
Reviewed-on: https://chromium-review.googlesource.com/1011885
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Cheng-Yu Lee <cylee@google.com>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550602}
[modify] https://crrev.com/8901493d98ea26516d5e794e5afba6f45d204c3d/chrome/browser/resource_coordinator/tab_lifecycle_unit.cc

Request to merge 8901493d98ea26516d5e794e5afba6f45d204c3d to M67 branch 3396. This commit fixes an important issue (tabs being discarded way to often), and landed just a little bit to late to be part of M67.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8901493d98ea26516d5e794e5afba6f45d204c3d

commit 8901493d98ea26516d5e794e5afba6f45d204c3d
Author: Kuo-Hsin Yang <vovoy@chromium.org>
Date: Fri Apr 13 13:00:30 2018

Fix Chrome OS tab discard regression

https://crrev.com/c/980742 replaces EstimatedMemoryFreedKB() with
GetEstimatedMemoryFreedOnDiscardKB() which always returns 0. It confuses
Chrome OS Tab Manager and all tabs except the front tab are discarded.
It's a quick fix to make tab discard work on Chrome OS.

Bug:  832517 
Change-Id: Ic72be7a7535bf6e0a48a6f6efa9afc28635813c8
Reviewed-on: https://chromium-review.googlesource.com/1011885
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Cheng-Yu Lee <cylee@google.com>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550602}
[modify] https://crrev.com/8901493d98ea26516d5e794e5afba6f45d204c3d/chrome/browser/resource_coordinator/tab_lifecycle_unit.cc

Do we know where this was reported?  M67 regression confirmed? Or a new feature since it's covering a //TODO?

I found this issue when I ran autotest power_LowMemorySuspend on M67 soraka. M67 regression confirmed.

Your change meets the bar and is auto-approved for M67. Please go ahead and merge the CL to branch 3396 manually. Please contact milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), kbleicher@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/792bf61cf22af38e200623e1e6b59e9166f90e1e

commit 792bf61cf22af38e200623e1e6b59e9166f90e1e
Author: Kuo-Hsin Yang <vovoy@chromium.org>
Date: Thu Apr 19 05:50:01 2018

[Merge to M67] Fix Chrome OS tab discard regression

https://crrev.com/c/980742 replaces EstimatedMemoryFreedKB() with
GetEstimatedMemoryFreedOnDiscardKB() which always returns 0. It confuses
Chrome OS Tab Manager and all tabs except the front tab are discarded.
It's a quick fix to make tab discard work on Chrome OS.

Bug:  832517 
Change-Id: Ic72be7a7535bf6e0a48a6f6efa9afc28635813c8
Reviewed-on: https://chromium-review.googlesource.com/1011885
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Cheng-Yu Lee <cylee@google.com>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#550602}(cherry picked from commit 8901493d98ea26516d5e794e5afba6f45d204c3d)
Reviewed-on: https://chromium-review.googlesource.com/1013800
Cr-Commit-Position: refs/branch-heads/3396@{#118}
Cr-Branched-From: 9ef2aa869bc7bc0c089e255d698cca6e47d6b038-refs/heads/master@{#550428}
[modify] https://crrev.com/792bf61cf22af38e200623e1e6b59e9166f90e1e/chrome/browser/resource_coordinator/tab_lifecycle_unit.cc

Vovo. How did you find this bug? Is there a test that can catch the bug?

The autotest power_LowMemorySuspend can catch this bug, I found this issue when I am developing power_LowMemorySuspend. power_LowMemorySuspend is a stress test that may run for more than 1 hour, I could add a simpler autotest to catch similar issue in the future.

I confirm that the issue was introduced by https://chromium-review.googlesource.com/c/chromium/src/+/980742, which landed in 67.0.3388.0, which is a refactor of an existing feature. The merge in comment #9 fixes the issue.