Add rudimentary mash shelf functionality.

components/mus/public/interfaces/BUILD.gn

 # Copyright 2014 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.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("interfaces") {
   sources = [
     "accelerator_registrar.mojom",
     "command_buffer.mojom",
     "compositor_frame.mojom",
     "cursor.mojom",
     "gpu.mojom",
     "gpu_capabilities.mojom",
     "input_event_constants.mojom",
     "input_event_matcher.mojom",
     "input_events.mojom",
     "input_key_codes.mojom",
     "mus_constants.mojom",
     "quads.mojom",
     "surface_id.mojom",
+    "user_window_controller.mojom",
     "window_manager.mojom",
     "window_manager_constants.mojom",
     "window_tree.mojom",
     "window_tree_host.mojom",
   ]
 
   import_dirs = [
     get_path_info("../../../..", "abspath"),
     "//mojo/services",
   ]
 
   deps = [
     "//ui/mojo/geometry:interfaces",
     "//ui/mojo/ime:interfaces",
   ]
 }