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This patch makes showing/hiding the WindowTreeHost's window behave the
same as if Widget Show/Hide was called. This is necessary for transient
windows to work. In particular it's expected that hiding the transient
parent hides transient children. TransientWindowManager does that by way
of calling show/hide directly on the aura window. I could have added the
ability to inject function to accomplish that, but I know I've run into
other code expecting show/hide on the aura window to map to showing/hiding
the widget.

BUG=883371
TEST=covered by tests

Change-Id: I6ee9009732ba1b5f47e888a1a841f462a1f5a851
Reviewed-on: https://chromium-review.googlesource.com/c/1340964
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610126}