Add rudimentary mash shelf functionality.

mash/wm/public/interfaces/user_window_controller.mojom

Index: mash/wm/public/interfaces/user_window_controller.mojom
diff --git a/mash/wm/public/interfaces/user_window_controller.mojom b/mash/wm/public/interfaces/user_window_controller.mojom
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+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module mash.wm.mojom;
+
+// An observer of user windows within mojom::CONTAINER_USER_WINDOWS.
+// TODO(msw): Observe focus changes, title/icon changes, etc.
+interface UserWindowObserver {
+  // Called when the observer is first added to supply the initial state.
+  OnUserWindowObserverAdded(array<uint32> window_ids);
+
+  OnUserWindowAdded(uint32 window_id);
+  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);
+};