Fix AshWindowTreeHost shutdown for Ozone X11.

ui/ozone/platform/headless/headless_window.cc

 // 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.
 
 #include "ui/ozone/platform/headless/headless_window.h"
 
 #include <string>
 
 #include "base/files/file_path.h"
 #include "base/strings/string_number_conversions.h"
@@ skipping 34 matching lines @@
 }
 
 void HeadlessWindow::SetTitle(const base::string16& title) {}
 
 void HeadlessWindow::Show() {}
 
 void HeadlessWindow::Hide() {}
 
 void HeadlessWindow::Close() {}
 
+void HeadlessWindow::PrepareForShutdown() {}
+
 void HeadlessWindow::SetCapture() {}
 
 void HeadlessWindow::ReleaseCapture() {}
 
 void HeadlessWindow::ToggleFullscreen() {}
 
 void HeadlessWindow::Maximize() {}
 
 void HeadlessWindow::Minimize() {}
 
 void HeadlessWindow::Restore() {}
 
 void HeadlessWindow::SetCursor(PlatformCursor cursor) {}
 
 void HeadlessWindow::MoveCursorTo(const gfx::Point& location) {}
 
 void HeadlessWindow::ConfineCursorToBounds(const gfx::Rect& bounds) {}
 
 PlatformImeController* HeadlessWindow::GetPlatformImeController() {
   return nullptr;
 }
 
 }  // namespace ui