See https://codereview.chromium.org/2033833002/ for details.

Code in //ui/message_center/views, specifically MessagePopupCollection, needs to
be able to parent windows to a container. It uses a NativeWindow for this. However, on mash we need to use a mus::Window container, even though NativeWindow is still an aura::Window.

The code is shared with desktop Chrome for showing notifications. Desktop Chrome does not provide a parent container, since these are shown over the desktop. I might be able to do something like:

* Pass an integer container_id into here.
* Add a client interface somewhere in //ui/views that can convert a
container_id into a void* parent.
* Depending on whether or not mus is running, cast the void* parent to
either an aura::Window (and assign to Widget::InitParams::parent) or to
a mus::Window (and assign to Widget::InitParams::parent_mus).

But that seems pretty distasteful.