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We have dependencies on TabletModeController and need to be destroyed
before it.

Previously we assumed that the view hierarchy would be destroyed before
our Assistant controllers. This will not actually be the case since I
am moving our reset above destruction of the view hierarchy.

As such, we now destroy our view hierarchy at the beginning of our
controller destruction sequence.

Also fixes some outdated comments.

Bug: b:112781572
Change-Id: Iecdce642790c0c8278d489734ea735370e98912c
Reviewed-on: https://chromium-review.googlesource.com/1180656
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584831}