Now that we're using aura this should map to aura::client::kModalKey.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/edab7eb543505b23c3618767d40e2aa4f4ddeb19

commit edab7eb543505b23c3618767d40e2aa4f4ddeb19
Author: moshayedi <moshayedi@chromium.org>
Date: Mon Mar 20 15:15:31 2017

Inform window manager about modal windows in mus+ash.

We need to inform window manager about modal windows. This is necessary because
for example SystemModalContainerLayoutManager needs to know about modal types
so it can create the fullscreen grey background for system modals.

After this change we have some of the WM effects for modal windows (e.g.
animating system modal windows when clicked outside of it), but for
the rest we need more wiring (e.g. no grey background yet) which will
be done in subsequent CLs.

BUG= 645996 

Review-Url: https://codereview.chromium.org/2745143004
Cr-Commit-Position: refs/heads/master@{#458065}

[modify] https://crrev.com/edab7eb543505b23c3618767d40e2aa4f4ddeb19/services/ui/public/interfaces/window_manager.mojom
[modify] https://crrev.com/edab7eb543505b23c3618767d40e2aa4f4ddeb19/services/ui/ws/test_utils.cc
[modify] https://crrev.com/edab7eb543505b23c3618767d40e2aa4f4ddeb19/services/ui/ws/test_utils.h
[modify] https://crrev.com/edab7eb543505b23c3618767d40e2aa4f4ddeb19/services/ui/ws/window_tree.cc
[modify] https://crrev.com/edab7eb543505b23c3618767d40e2aa4f4ddeb19/services/ui/ws/window_tree_client_unittest.cc
[modify] https://crrev.com/edab7eb543505b23c3618767d40e2aa4f4ddeb19/services/ui/ws/window_tree_unittest.cc
[modify] https://crrev.com/edab7eb543505b23c3618767d40e2aa4f4ddeb19/ui/aura/mus/window_tree_client.cc
[modify] https://crrev.com/edab7eb543505b23c3618767d40e2aa4f4ddeb19/ui/aura/mus/window_tree_client.h