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}
3 files changed