And it is being reverted as we speak:
https://chromium-review.googlesource.com/c/chromium/src/+/1376137

The work is assigned/started, so removing sheriff label.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/77fbe34018de545f5b733709a2afcb1391ef1364

commit 77fbe34018de545f5b733709a2afcb1391ef1364
Author: Xiyuan Xia <xiyuan@chromium.org>
Date: Thu Dec 13 23:44:02 2018

Reland "wm: Update nested activation handling"

This is a reland of ecde845099001740fee119a82d45664e622d0ade

Original change's description:
> wm: Update nested activation handling
>
> - NOTREACHED() and no-ope if the nested activation wants to change
>   to a different active window other than the one being activated;
> - Allow focus shift in nested activations as long as they don't
>   change the window to be activated
>
> Bug:  908524 
> Test: FocusControllerApiTest.ActivateWhileActivating
> Change-Id: I105c85bf2671c7e396fdcbedff7b8082f007f642
> Reviewed-on: https://chromium-review.googlesource.com/c/1357017
> Reviewed-by: Scott Violet <sky@chromium.org>
> Reviewed-by: Mitsuru Oshima (gardener - slow) <oshima@chromium.org>
> Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#616346}

Bug:  908524 ,  914915 
Change-Id: Id0072941bfb0def54f093bcb771b2d8485c9b04e
Reviewed-on: https://chromium-review.googlesource.com/c/1376726
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616496}
[modify] https://crrev.com/77fbe34018de545f5b733709a2afcb1391ef1364/ui/wm/core/focus_controller.cc
[modify] https://crrev.com/77fbe34018de545f5b733709a2afcb1391ef1364/ui/wm/core/focus_controller.h
[modify] https://crrev.com/77fbe34018de545f5b733709a2afcb1391ef1364/ui/wm/core/focus_controller_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7762ed06e63d2622c869d179ecfb8674441711e8

commit 7762ed06e63d2622c869d179ecfb8674441711e8
Author: Hitoshi Yoshida <peria@chromium.org>
Date: Fri Dec 14 01:39:56 2018

Revert "Reland "wm: Update nested activation handling""

This reverts commit 77fbe34018de545f5b733709a2afcb1391ef1364.

Reason for revert: A test still fails with this.
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20TSan%20Tests/31030

Original change's description:
> Reland "wm: Update nested activation handling"
> 
> This is a reland of ecde845099001740fee119a82d45664e622d0ade
> 
> Original change's description:
> > wm: Update nested activation handling
> >
> > - NOTREACHED() and no-ope if the nested activation wants to change
> >   to a different active window other than the one being activated;
> > - Allow focus shift in nested activations as long as they don't
> >   change the window to be activated
> >
> > Bug:  908524 
> > Test: FocusControllerApiTest.ActivateWhileActivating
> > Change-Id: I105c85bf2671c7e396fdcbedff7b8082f007f642
> > Reviewed-on: https://chromium-review.googlesource.com/c/1357017
> > Reviewed-by: Scott Violet <sky@chromium.org>
> > Reviewed-by: Mitsuru Oshima (gardener - slow) <oshima@chromium.org>
> > Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#616346}
> 
> Bug:  908524 ,  914915 
> Change-Id: Id0072941bfb0def54f093bcb771b2d8485c9b04e
> Reviewed-on: https://chromium-review.googlesource.com/c/1376726
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#616496}

TBR=xiyuan@chromium.org,sky@chromium.org,oshima@chromium.org

Change-Id: I8fd98eca46991af0cd6a780f44f7912c5d6bad13
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug:  908524 ,  914915 
Reviewed-on: https://chromium-review.googlesource.com/c/1377477
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616552}
[modify] https://crrev.com/7762ed06e63d2622c869d179ecfb8674441711e8/ui/wm/core/focus_controller.cc
[modify] https://crrev.com/7762ed06e63d2622c869d179ecfb8674441711e8/ui/wm/core/focus_controller.h
[modify] https://crrev.com/7762ed06e63d2622c869d179ecfb8674441711e8/ui/wm/core/focus_controller_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fd8ad21305da505a82ed7ab6dc3c49738eaa802e

commit fd8ad21305da505a82ed7ab6dc3c49738eaa802e
Author: Xiyuan Xia <xiyuan@chromium.org>
Date: Fri Dec 14 18:27:28 2018

Reland "Reland "wm: Update nested activation handling""

This is a reland of 77fbe34018de545f5b733709a2afcb1391ef1364
with fix for the UAF.

Original change's description:
> Reland "wm: Update nested activation handling"
>
> This is a reland of ecde845099001740fee119a82d45664e622d0ade
>
> Original change's description:
> > wm: Update nested activation handling
> >
> > - DCHECK() and no-op if the nested activation wants to change
> >   to a different active window other than the one being activated;
> > - Allow focus shift in nested activations as long as they don't
> >   change the window to be activated
> >
> > Bug:  908524 
> > Test: FocusControllerApiTest.ActivateWhileActivating
> > Change-Id: I105c85bf2671c7e396fdcbedff7b8082f007f642
> > Reviewed-on: https://chromium-review.googlesource.com/c/1357017
> > Reviewed-by: Scott Violet <sky@chromium.org>
> > Reviewed-by: Mitsuru Oshima (gardener - slow) <oshima@chromium.org>
> > Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#616346}
>
> Bug:  908524 ,  914915 
> Change-Id: Id0072941bfb0def54f093bcb771b2d8485c9b04e
> Reviewed-on: https://chromium-review.googlesource.com/c/1376726
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#616496}

Bug:  908524 ,  914915 
Change-Id: Id3786796d109cfdb7c4caf1529243af93830da69
Reviewed-on: https://chromium-review.googlesource.com/c/1378236
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616751}
[modify] https://crrev.com/fd8ad21305da505a82ed7ab6dc3c49738eaa802e/ui/wm/core/focus_controller.cc
[modify] https://crrev.com/fd8ad21305da505a82ed7ab6dc3c49738eaa802e/ui/wm/core/focus_controller.h
[modify] https://crrev.com/fd8ad21305da505a82ed7ab6dc3c49738eaa802e/ui/wm/core/focus_controller_unittest.cc