Makes display roots be destroyed by client

ash/mus/window_manager.cc

 // 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.
 
 #include "ash/mus/window_manager.h"
 
 #include <stdint.h>
 
 #include <utility>
 
@@ skipping 279 matching lines @@
     bool janky) {
   for (auto* window : client_windows)
     SetWindowIsJanky(window, janky);
 }
 
 void WindowManager::OnWmNewDisplay(ui::Window* window,
                                    const display::Display& display) {
   CreateRootWindowController(window, display);
 }
 
+void WindowManager::OnWmDisplayRemoved(ui::Window* window) {
+  // TODO(sky): wire up this up correctly.
+  NOTIMPLEMENTED();
+}
+
 void WindowManager::OnWmPerformMoveLoop(
     ui::Window* window,
     ui::mojom::MoveLoopSource source,
     const gfx::Point& cursor_location,
     const base::Callback<void(bool)>& on_done) {
   WmWindowMus* child_window = WmWindowMus::Get(window);
   MoveEventHandler* handler = MoveEventHandler::GetForWindow(child_window);
   if (!handler) {
     on_done.Run(false);
     return;
@@ skipping 18 matching lines @@
                                                     const ui::Event& event) {
   auto iter = accelerator_handlers_.find(GetAcceleratorNamespaceId(id));
   if (iter == accelerator_handlers_.end())
     return ui::mojom::EventResult::HANDLED;
 
   return iter->second->OnAccelerator(id, event);
 }
 
 }  // namespace mus
 }  // namespace ash