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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}