This is a dependency of the status area widget which I'm trying to convert to
wm common types.

Introduce a method to the message center's delegate that allows it to
compute the parent container for a popup. This cannot be a NativeView, since
on mus a NativeView is an aura::Window, but the parent container is a mus
window.

This can't move to //ash/common yet because of system tray dependencies.

BUG=619636
TEST=ash_unittests

Review-Url: https://codereview.chromium.org/2075953003
Cr-Commit-Position: refs/heads/master@{#400738}