This is a reland of b650c38e
Original change's description:
> chromeos: fix DCHECK in ~WindowTreeHostManager
> 
> The DCHECK is because on observer is being added but not
> removed. Specifically exo's WmHelper installs an observer on
> WindowTreeHostManager but WmHelper never removes itself (because
> WindowTreeHostManager is deleted before WmHelper). This patch makes
> WmHelper be destroyed before WindowTreeHostManager by way of making
> ChromeBrowserMainExtraPartsAsh create exo related functionality.
> 
> BUG=none
> TEST=none
> 
> Change-Id: I84e7209c817e4687017edbc14db7f75cb1054fbb
> Reviewed-on: https://chromium-review.googlesource.com/572427


> Commit-Queue: Scott Violet <sky@chromium.org>
> Reviewed-by: David Reveman <reveman@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#487556}

Bug: none
Change-Id: I486d68e9a2af654facd50ccb0512bdca217520a8
Reviewed-on: https://chromium-review.googlesource.com/578287


Reviewed-by: David Reveman <reveman@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488292}