Changes how window manager obtains WindowTree from mus

components/mus/public/interfaces/window_manager_window_tree_factory.mojom

+// Copyright 2016 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 mus.mojom;
+
+import "components/mus/public/interfaces/window_manager_constants.mojom";
+import "components/mus/public/interfaces/window_tree.mojom";
+
+// Interface used by the WindowManager to obtain a WindowTree. The
+// WindowManager is informed of the roots (one per display) by way of
+// WmNewDisplayAdded(). See it for details.
+interface WindowManagerWindowTreeFactory {
+  // NOTE: it is expected this is called only once.
+  CreateWindowTree(WindowTree& tree_request, WindowTreeClient client);
+};