Add rudimentary mash shelf functionality.

components/mus/public/interfaces/user_window_controller.mojom

Index: components/mus/public/interfaces/user_window_controller.mojom
diff --git a/components/mus/public/interfaces/user_window_controller.mojom b/components/mus/public/interfaces/user_window_controller.mojom
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index 0000000000000000000000000000000000000000..b456749cb3369439a45e74a83796620ffeee9f76
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+// Copyright 2015 The Chromium Authors. All rights reserved.

[sky, 2016/01/13 21:17:17]

2016.
Also, I don't think this interface should be here. UserWindowController isn't
implemented by mus or depended on by a layer that requires it to be here. I
recommend mash/wm/public/interfaces.

[msw, 2016/01/13 22:54:56]

Done.

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module mus.mojom;
+
+// An observer of user windows within mojom::CONTAINER_USER_WINDOWS.
+// TODO(msw): Observe focus changes, title/icon changes, etc.
+interface UserWindowObserver {
+  OnUserWindowAdded(uint32 window_id);

[sky, 2016/01/13 21:17:17]

I recommend an initial function that is always called with the current state.
That way the observer can stay up to date and there aren't ordering problems if
you have a query interface.

[msw, 2016/01/13 22:54:56]

Done; this gets the quick launcher window now too!
(I hope I captured your intent correctly)

+  OnUserWindowRemoved(uint32 window_id);
+};
+
+// An interface allowing system UIs to manage the set of user windows.
+// TODO(msw): Add minimization, restoration, opening a chooser view, etc.
+interface UserWindowController {
+  AddUserWindowObserver(UserWindowObserver observer);
+  FocusUserWindow(uint32 window_id);
+};