Reverting r431482 seems to fix the failure on trybot:
https://codereview.chromium.org/2493163002/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fb027e734b0b86cbe440c70d6a62f63fa5c4b7b9

commit fb027e734b0b86cbe440c70d6a62f63fa5c4b7b9
Author: hiroshige <hiroshige@chromium.org>
Date: Fri Nov 11 11:44:54 2016

Revert of Fix bug where removed (but not deleted) windows are not reflected in the tree properly (patchset #7 id:120001 of https://codereview.chromium.org/2476353002/ )

Reason for revert:
Suspected to cause SystemTrayTest.NullDefaultViewIsNotRecorded failure.

BUG= 664428 

Original issue's description:
> Fix bug where removed (but not deleted) windows are not reflected in the tree properly
>
> When there are windows which are removed as children of some parent, and
> they still exist, OnWindowDestroying is not called because the child window
> was never destroyed. However, OnWindowTreeChanging is called, if
> the target has an old_parent, aka the parent it is being removed from.
> OnWindowTreeChanged is called if the target has a new_parent, aka the
> parent it is being added to.
>
> This CL removes nodes on OnWindowTreeChanging and adds them back in
> OnWindowTreeChanged if necessary. Since we only want them to trigger once,
> we check to see in either callback if the window that the callback is
> being called on is the old_parent or the new_parent.
>
> BUG=648701
>
> Committed: https://crrev.com/dbef379ab2b2c8a2127d20839f77d355e9f5742e
> Cr-Commit-Position: refs/heads/master@{#431482}

TBR=sadrul@chromium.org,mhashmi@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=648701

Review-Url: https://codereview.chromium.org/2498593002
Cr-Commit-Position: refs/heads/master@{#431541}

[modify] https://crrev.com/fb027e734b0b86cbe440c70d6a62f63fa5c4b7b9/ash/common/devtools/ash_devtools_dom_agent.cc
[modify] https://crrev.com/fb027e734b0b86cbe440c70d6a62f63fa5c4b7b9/ash/common/devtools/ash_devtools_dom_agent.h
[modify] https://crrev.com/fb027e734b0b86cbe440c70d6a62f63fa5c4b7b9/ash/common/devtools/ash_devtools_unittest.cc

After r431482 is reverted, ash_unittests stops failing:
https://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20%281%29/builds/59847

r431481 is already relanded.
r431480 is going to be relanded.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7b3c74117576554ff2c044b12e5d8c8937c9d5d7

commit 7b3c74117576554ff2c044b12e5d8c8937c9d5d7
Author: hiroshige <hiroshige@chromium.org>
Date: Fri Nov 11 12:35:59 2016

Reland of Correct spacing in scroll contents of Ash MD detailed view. (patchset #1 id:1 of https://codereview.chromium.org/2500433002/ )

Reason for revert:
Not relevant to the test failure.
BUG= 664428 

Original issue's description:
> Revert of Correct spacing in scroll contents of Ash MD detailed view. (patchset #1 id:1 of https://codereview.chromium.org/2484913006/ )
>
> Reason for revert:
> Suspected to cause SystemTrayTest.NullDefaultViewIsNotRecorded (ash_unittests) failure.
> https://build.chromium.org/p/chromium.win/builders/Win7%20Tests%20(1)/builds/59826
>
> according to keywords "system" "tray" "views" in the description.
>
> Original issue's description:
> > Correct spacing in scroll contents of Ash MD detailed view.
> >
> > This CL adds 4px vertical spaceing for scroll contents in ScrollContentsViewto
> > ensure that there is a correct padding between top row and scroll contents.
> > This also removes spacing between children in ScrollContentsView to comply with
> > the latest MD design spec.
> >
> > BUG=632102,663199,663457,663461,663154,632025,632115
> > TEST=manually verified the layout.
> >
> > Committed: https://crrev.com/bb150edfa1495353882b799b39e05b0d181bcd9c
> > Cr-Commit-Position: refs/heads/master@{#431480}
>
> TBR=tdanderson@chromium.org,fukino@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=632102,663199,663457,663461,663154,632025,632115
>
> Committed: https://crrev.com/1137d16544665bb926542a25c986f0750433caa3
> Cr-Commit-Position: refs/heads/master@{#431517}

TBR=tdanderson@chromium.org,fukino@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=632102,663199,663457,663461,663154,632025,632115

Review-Url: https://codereview.chromium.org/2492833004
Cr-Commit-Position: refs/heads/master@{#431548}

[modify] https://crrev.com/7b3c74117576554ff2c044b12e5d8c8937c9d5d7/ash/common/system/tray/tray_details_view.cc